build: enable missing OpenSSF-recommended warnings, with fixes

https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.html
as of 2023-11-29 [1].

Enable new recommended warnings (except `-Wsign-conversion`):

- enable `-Wformat=2` for clang (in both cmake and autotools).
- add `CURL_PRINTF()` internal attribute and mark functions accepting
  printf arguments with it. This is a copy of existing
  `CURL_TEMP_PRINTF()` but using `__printf__` to make it compatible
  with redefinting the `printf` symbol:
  https://gcc.gnu.org/onlinedocs/gcc-3.0.4/gcc_5.html#SEC94
- fix `CURL_PRINTF()` and existing `CURL_TEMP_PRINTF()` for
  mingw-w64 and enable it on this platform.
- enable `-Wimplicit-fallthrough`.
- enable `-Wtrampolines`.
- add `-Wsign-conversion` commented with a FIXME.
- cmake: enable `-pedantic-errors` the way we do it with autotools.
  Follow-up to d5c0351055 #2747
- lib/curl_trc.h: use `CURL_FORMAT()`, this also fixes it to enable format
  checks. Previously it was always disabled due to the internal `printf`
  macro.

Fix them:

- fix bug where an `set_ipv6_v6only()` call was missed in builds with
  `--disable-verbose` / `CURL_DISABLE_VERBOSE_STRINGS=ON`.
- add internal `FALLTHROUGH()` macro.
- replace obsolete fall-through comments with `FALLTHROUGH()`.
- fix fallthrough markups: Delete redundant ones (showing up as
  warnings in most cases). Add missing ones. Fix indentation.
- silence `-Wformat-nonliteral` warnings with llvm/clang.
- fix one `-Wformat-nonliteral` warning.
- fix new `-Wformat` and `-Wformat-security` warnings.
- fix `CURL_FORMAT_SOCKET_T` value for mingw-w64. Also move its
  definition to `lib/curl_setup.h` allowing use in `tests/server`.
- lib: fix two wrongly passed string arguments in log outputs.
  Co-authored-by: Jay Satiro
- fix new `-Wformat` warnings on mingw-w64.

[1] 56c0fde389/docs/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C%2B%2B.md

Closes #12489

lib/vssh/libssh.c

@@ -439,11 +439,8 @@ static int myssh_is_known(struct Curl_easy *data)
       keymatch = CURLKHMATCH_OK;
       break;
     case SSH_KNOWN_HOSTS_OTHER:
-      /* fallthrough */
     case SSH_KNOWN_HOSTS_NOT_FOUND:
-      /* fallthrough */
     case SSH_KNOWN_HOSTS_UNKNOWN:
-      /* fallthrough */
     case SSH_KNOWN_HOSTS_ERROR:
       keymatch = CURLKHMATCH_MISSING;
       break;
@@ -459,7 +456,6 @@ static int myssh_is_known(struct Curl_easy *data)
       keymatch = CURLKHMATCH_OK;
       break;
     case SSH_SERVER_FILE_NOT_FOUND:
-      /* fallthrough */
     case SSH_SERVER_NOT_KNOWN:
       keymatch = CURLKHMATCH_MISSING;
       break;
@@ -623,7 +619,7 @@ restart:
       if(rc < 0)
         return SSH_ERROR;
 
-    /* FALLTHROUGH */
+      FALLTHROUGH();
     case 1:
       sshc->kbd_state = 1;
 
@@ -698,7 +694,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       ssh_set_blocking(sshc->ssh_session, 0);
 
       state(data, SSH_S_STARTUP);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
 
     case SSH_S_STARTUP:
       rc = ssh_connect(sshc->ssh_session);
@@ -713,7 +709,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
 
       state(data, SSH_HOSTKEY);
 
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_HOSTKEY:
 
       rc = myssh_is_known(data);
@@ -723,7 +719,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       }
 
       state(data, SSH_AUTHLIST);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_AUTHLIST:{
         sshc->authed = FALSE;
 
@@ -904,7 +900,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
         break;
       }
       state(data, SSH_AUTH_PASS);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
 
     case SSH_AUTH_PASS:
       rc = ssh_userauth_password(sshc->ssh_session, NULL, conn->passwd);
@@ -967,7 +963,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
         break;
       }
       state(data, SSH_SFTP_REALPATH);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_SFTP_REALPATH:
       /*
        * Get the "home" directory
@@ -1554,7 +1550,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       sshc->readdir_longentry = NULL;
 
       state(data, SSH_SFTP_READDIR_BOTTOM);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_SFTP_READDIR_BOTTOM:
       if(Curl_dyn_addn(&sshc->readdir_buf, "\n", 1))
         result = CURLE_OUT_OF_MEMORY;
@@ -1878,7 +1874,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
         break;
       }
       state(data, SSH_SCP_DOWNLOAD);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
 
     case SSH_SCP_DOWNLOAD:{
         curl_off_t bytecount;
@@ -1942,7 +1938,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       ssh_set_blocking(sshc->ssh_session, 0);
 
       state(data, SSH_SESSION_DISCONNECT);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
 
     case SSH_SESSION_DISCONNECT:
       /* during weird times when we've been prematurely aborted, the channel
@@ -1965,7 +1961,7 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       data->state.most_recent_ftp_entrypath = NULL;
 
       state(data, SSH_SESSION_FREE);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_SESSION_FREE:
       if(sshc->ssh_session) {
         ssh_free(sshc->ssh_session);
@@ -2016,7 +2012,6 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
       break;
 
     case SSH_QUIT:
-      /* fallthrough, just stop! */
     default:
       /* internal error */
       sshc->nextstate = SSH_NO_STATE;
@@ -2607,7 +2602,7 @@ static ssize_t sftp_recv(struct Curl_easy *data, int sockindex,
         return -1;
       }
 
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case 1:
       conn->proto.sshc.sftp_recv_state = 1;
 

lib/vssh/libssh2.c

@@ -589,10 +589,9 @@ static CURLcode ssh_knownhost(struct Curl_easy *data)
 
     switch(rc) {
     default: /* unknown return codes will equal reject */
-      /* FALLTHROUGH */
     case CURLKHSTAT_REJECT:
       state(data, SSH_SESSION_FREE);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case CURLKHSTAT_DEFER:
       /* DEFER means bail out but keep the SSH_HOSTKEY state */
       result = sshc->actualcode = CURLE_PEER_FAILED_VERIFICATION;
@@ -601,9 +600,8 @@ static CURLcode ssh_knownhost(struct Curl_easy *data)
       /* remove old host+key that doesn't match */
       if(host)
         libssh2_knownhost_del(sshc->kh, host);
-        /* FALLTHROUGH */
+      FALLTHROUGH();
     case CURLKHSTAT_FINE:
-        /* FALLTHROUGH */
     case CURLKHSTAT_FINE_ADD_TO_FILE:
       /* proceed */
       if(keycheck != LIBSSH2_KNOWNHOST_CHECK_MATCH) {
@@ -997,7 +995,7 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
       }
 
       state(data, SSH_S_STARTUP);
-      /* FALLTHROUGH */
+      FALLTHROUGH();
 
     case SSH_S_STARTUP:
       rc = session_startup(sshc->ssh_session, sock);
@@ -1016,7 +1014,7 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
 
       state(data, SSH_HOSTKEY);
 
-      /* FALLTHROUGH */
+      FALLTHROUGH();
     case SSH_HOSTKEY:
       /*
        * Before we authenticate we should check the hostkey's fingerprint
@@ -3024,7 +3022,6 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
       break;
 
     case SSH_QUIT:
-      /* fallthrough, just stop! */
     default:
       /* internal error */
       sshc->nextstate = SSH_NO_STATE;