build: fix some -Wsign-conversion/-Warith-conversion warnings

- enable `-Wsign-conversion` warnings, but also setting them to not
  raise errors.
- fix `-Warith-conversion` warnings seen in CI.
  These are triggered by `-Wsign-converion` and causing errors unless
  explicitly silenced. It makes more sense to fix them, there just a few
  of them.
- fix some `-Wsign-conversion` warnings.
- hide `-Wsign-conversion` warnings with a `#pragma`.
- add macro `CURL_WARN_SIGN_CONVERSION` to unhide them on a per-build
  basis.
- update a CI job to unhide them with the above macro:
  https://github.com/curl/curl/actions/workflows/linux.yml -> OpenSSL -O3

Closes #12492

tests/server/mqttd.c

@@ -569,7 +569,7 @@ static curl_socket_t mqttit(curl_socket_t fd)
         goto end;
       }
       /* ignore the connect flag byte and two keepalive bytes */
-      payload_len = (buffer[10] << 8) | buffer[11];
+      payload_len = (size_t)(buffer[10] << 8) | buffer[11];
       /* first part of the payload is the client ID */
       client_id_length = payload_len;
 
@@ -579,14 +579,16 @@ static curl_socket_t mqttit(curl_socket_t fd)
       start_usr = client_id_offset + payload_len;
       if(usr_flag == (unsigned char)(conn_flags & usr_flag)) {
         logmsg("User flag is present in CONN flag");
-        payload_len += (buffer[start_usr] << 8) | buffer[start_usr + 1];
+        payload_len += (size_t)(buffer[start_usr] << 8) |
+                       buffer[start_usr + 1];
         payload_len += 2; /* MSB and LSB for user length */
       }
 
       start_passwd = client_id_offset + payload_len;
       if(passwd_flag == (char)(conn_flags & passwd_flag)) {
         logmsg("Password flag is present in CONN flags");
-        payload_len += (buffer[start_passwd] << 8) | buffer[start_passwd + 1];
+        payload_len += (size_t)(buffer[start_passwd] << 8) |
+                       buffer[start_passwd + 1];
         payload_len += 2; /* MSB and LSB for password length */
       }
 
@@ -631,7 +633,7 @@ static curl_socket_t mqttit(curl_socket_t fd)
       packet_id = (unsigned short)((buffer[0] << 8) | buffer[1]);
 
       /* two bytes topic length */
-      topic_len = (buffer[2] << 8) | buffer[3];
+      topic_len = (size_t)(buffer[2] << 8) | buffer[3];
       if(topic_len != (remaining_length - 5)) {
         logmsg("Wrong topic length, got %u expected %zu",
                topic_len, remaining_length - 5);
@@ -676,7 +678,7 @@ static curl_socket_t mqttit(curl_socket_t fd)
       logprotocol(FROM_CLIENT, "PUBLISH", remaining_length,
                   dump, buffer, rc);
 
-      topiclen = (buffer[1 + bytes] << 8) | buffer[2 + bytes];
+      topiclen = (size_t)(buffer[1 + bytes] << 8) | buffer[2 + bytes];
       logmsg("Got %zu bytes topic", topiclen);
       /* TODO: verify topiclen */
 

tests/server/util.c

@@ -137,7 +137,7 @@ void logmsg(const char *msg, ...)
 static const char *win32_strerror(int err, char *buf, size_t buflen)
 {
   if(!FormatMessageA((FORMAT_MESSAGE_FROM_SYSTEM |
-                      FORMAT_MESSAGE_IGNORE_INSERTS), NULL, err,
+                      FORMAT_MESSAGE_IGNORE_INSERTS), NULL, (DWORD)err,
                      LANG_NEUTRAL, buf, (DWORD)buflen, NULL))
     msnprintf(buf, buflen, "Unknown error %d (%#x)", err, err);
   return buf;
@@ -247,7 +247,7 @@ int wait_ms(int timeout_ms)
 #if defined(MSDOS)
   delay(timeout_ms);
 #elif defined(USE_WINSOCK)
-  Sleep(timeout_ms);
+  Sleep((DWORD)timeout_ms);
 #else
   pending_ms = timeout_ms;
   initial_tv = tvnow();