build: add more picky warnings and fix them

Enable more picky compiler warnings. I've found these options in the
nghttp3 project when implementing the CMake quick picky warning
functionality for it [1].

`-Wunused-macros` was too noisy to keep around, but fixed a few issues
it revealed while testing.

- autotools: reflect the more precisely-versioned clang warnings.
  Follow-up to 033f8e2a08 #12324
- autotools: sync between clang and gcc the way we set `no-multichar`.
- autotools: avoid setting `-Wstrict-aliasing=3` twice.
- autotools: disable `-Wmissing-noreturn` for MSYS gcc targets [2].
  It triggers in libtool-generated stub code.

- lib/timeval: delete a redundant `!MSDOS` guard from a `WIN32` branch.

- lib/curl_setup.h: delete duplicate declaration for `fileno`.
  Added in initial commit ae1912cb0d
  (1999-12-29). This suggests this may not be needed anymore, but if
  it does, we may restore this for those specific (non-Windows) systems.
- lib: delete unused macro `FTP_BUFFER_ALLOCSIZE` since
  c1d6fe2aaa.
- lib: delete unused macro `isxdigit_ascii` since
  f65f750742.
- lib/mqtt: delete unused macro `MQTT_HEADER_LEN`.
- lib/multi: delete unused macro `SH_READ`/`SH_WRITE`.
- lib/hostip: add `noreturn` function attribute via new `CURL_NORETURN`
  macro.
- lib/mprintf: delete duplicate declaration for `Curl_dyn_vprintf`.
- lib/rand: fix `-Wunreachable-code` and related fallouts [3].
- lib/setopt: fix `-Wunreachable-code-break`.
- lib/system_win32 and lib/timeval: fix double declarations for
  `Curl_freq` and `Curl_isVistaOrGreater` in CMake UNITY mode [4].
- lib/warnless: fix double declarations in CMake UNITY mode [5].
  This was due to force-disabling the header guard of `warnless.h` to
  to reapply it to source code coming after `warnless.c` in UNITY
  builds. This reapplied declarations too, causing the warnings.
  Solved by adding a header guard for the lines that actually need
  to be reapplied.
- lib/vauth/digest: fix `-Wunreachable-code-break` [6].
- lib/vssh/libssh2: fix `-Wunreachable-code-break` and delete redundant
  block.
- lib/vtls/sectransp: fix `-Wunreachable-code-break` [7].
- lib/vtls/sectransp: suppress `-Wunreachable-code`.
  Detected in `else` branches of dynamic feature checks, with results
  known at compile-time, e.g.
  ```c
  if(SecCertificateCopySubjectSummary)  /* -> true */
  ```
  Likely fixable as a separate micro-project, but given SecureTransport
  is deprecated anyway, let's just silence these locally.
- src/tool_help: delete duplicate declaration for `helptext`.
- src/tool_xattr: fix `-Wunreachable-code`.
- tests: delete duplicate declaration for `unitfail` [8].
- tests: delete duplicate declaration for `strncasecompare`.
- tests/libtest: delete duplicate declaration for `gethostname`.
  Originally added in 687df5c8c3
  (2010-08-02).
  Got complicated later: c49e9683b8
  If there are still systems around with warnings, we may restore the
  prototype, but limited for those systems.
- tests/lib2305: delete duplicate declaration for
  `libtest_debug_config`.
- tests/h2-download: fix `-Wunreachable-code-break`.

[1] a70edb08e9/cmake/PickyWarningsC.cmake
[2] https://ci.appveyor.com/project/curlorg/curl/builds/48553586/job/3qkgjauiqla5fj45?fullLog=true#L1675
[3] https://github.com/curl/curl/actions/runs/6880886309/job/18716044703?pr=12331#step:7:72
    https://github.com/curl/curl/actions/runs/6883016087/job/18722707368?pr=12331#step:7:109
[4] https://ci.appveyor.com/project/curlorg/curl/builds/48555101/job/9g15qkrriklpf1ut#L204
[5] https://ci.appveyor.com/project/curlorg/curl/builds/48555101/job/9g15qkrriklpf1ut#L218
[6] https://github.com/curl/curl/actions/runs/6880886309/job/18716042927?pr=12331#step:7:290
[7] https://github.com/curl/curl/actions/runs/6891484996/job/18746659406?pr=12331#step:9:1193
[8] https://github.com/curl/curl/actions/runs/6882803986/job/18722082562?pr=12331#step:33:1870

Closes #12331

lib/rand.c

@@ -101,7 +101,6 @@ CURLcode Curl_win32_random(unsigned char *entropy, size_t length)
 
 static CURLcode randit(struct Curl_easy *data, unsigned int *rnd)
 {
-  unsigned int r;
   CURLcode result = CURLE_OK;
   static unsigned int randseed;
   static bool seeded = FALSE;
@@ -143,8 +142,10 @@ static CURLcode randit(struct Curl_easy *data, unsigned int *rnd)
 #endif
 
 #if defined(HAVE_ARC4RANDOM) && !defined(USE_OPENSSL)
-  *rnd = (unsigned int)arc4random();
-  return CURLE_OK;
+  if(!seeded) {
+    *rnd = (unsigned int)arc4random();
+    return CURLE_OK;
+  }
 #endif
 
 #if defined(RANDOM_FILE) && !defined(WIN32)
@@ -171,9 +172,12 @@ static CURLcode randit(struct Curl_easy *data, unsigned int *rnd)
     seeded = TRUE;
   }
 
-  /* Return an unsigned 32-bit pseudo-random number. */
-  r = randseed = randseed * 1103515245 + 12345;
-  *rnd = (r << 16) | ((r >> 16) & 0xFFFF);
+  {
+    unsigned int r;
+    /* Return an unsigned 32-bit pseudo-random number. */
+    r = randseed = randseed * 1103515245 + 12345;
+    *rnd = (r << 16) | ((r >> 16) & 0xFFFF);
+  }
   return CURLE_OK;
 }