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

include/curl/mprintf.h

@@ -34,10 +34,16 @@ extern "C" {
 
 #if (defined(__GNUC__) || defined(__clang__)) &&                        \
   defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) &&         \
-  !defined(__MINGW32__) && !defined(CURL_NO_FMT_CHECKS)
-#define CURL_TEMP_PRINTF(a,b) __attribute__ ((format(printf, a, b)))
+  !defined(CURL_NO_FMT_CHECKS)
+#if defined(__MINGW32__) && !defined(__clang__)
+#define CURL_TEMP_PRINTF(fmt, arg) \
+  __attribute__((format(gnu_printf, fmt, arg)))
 #else
-#define CURL_TEMP_PRINTF(a,b)
+#define CURL_TEMP_PRINTF(fmt, arg) \
+  __attribute__((format(printf, fmt, arg)))
+#endif
+#else
+#define CURL_TEMP_PRINTF(fmt, arg)
 #endif
 
 CURL_EXTERN int curl_mprintf(const char *format, ...) CURL_TEMP_PRINTF(1, 2);