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 d5c0351055d5709da8f3e16c91348092fdb481aa #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] https://github.com/ossf/wg-best-practices-os-developers/blob/56c0fde3895bfc55c8a973ef49a2572c507b2ae1/docs/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C%2B%2B.md

Closes #12489

1 files changed, 2 insertions, 3 deletions

diff --git a/docs/examples/multi-debugcallback.c b/docs/examples/multi-debugcallback.c
index 8bdb58800..10842e950 100644
--- a/docs/examples/multi-debugcallback.c
+++ b/docs/examples/multi-debugcallback.c
@@ -100,10 +100,7 @@ int my_trace(CURL *handle, curl_infotype type,
   switch(type) {
   case CURLINFO_TEXT:
     fprintf(stderr, "== Info: %s", data);
-    /* FALLTHROUGH */
-  default: /* in case a new one is introduced to shock us */
     return 0;
-
   case CURLINFO_HEADER_OUT:
     text = "=> Send header";
     break;
@@ -116,6 +113,8 @@ int my_trace(CURL *handle, curl_infotype type,
   case CURLINFO_DATA_IN:
     text = "<= Recv data";
     break;
+  default: /* in case a new one is introduced to shock us */
+    return 0;
   }
 
   dump(text, stderr, data, size, TRUE);