checksrc: reduce exceptions, apply again to curlx

- tests/libtest: move exception to `stub_gssapi.h`.
- tests/libtest: move remaining exception to `testtrace.c`.
- tests/server: drop obsolete exception.
- docs/examples: move `BANNEDFUNC` exceptions to local files (3 lines).
- docs/examples: move `ERRNOVAR` exception to `ephiperfifo.c`.
- docs/examples: drop `typedef struct` (8 files).
- lib/curlx: add `.checksrc` with banned funcs copied from lib.
- checksrc: ban `strncpy`, `strtok_r`, `strtoul` by default.
  Drop local bans. Add exception for `strtoul` to `tests/server'.
- lib, src: sync banned funcs.

Also:
- REUSE: drop `stunnel.pem`, it no longer exists.
- docs/examples: formatting.
- docs/examples: simplify some `sizeof()`s.

Closes #17764

docs/examples/http2-upload.c

@@ -158,6 +158,7 @@ int my_trace(CURL *handle, curl_infotype type,
     known_offset = 1;
   }
   secs = epoch_offset + tv.tv_sec;
+  /* !checksrc! disable BANNEDFUNC 1 */
   now = localtime(&secs);  /* not thread safe but we do not care */
   curl_msnprintf(timebuf, sizeof(timebuf), "%02d:%02d:%02d.%06ld",
                  now->tm_hour, now->tm_min, now->tm_sec, (long)tv.tv_usec);