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

scripts/checksrc.pl

@@ -56,6 +56,9 @@ my %banfunc = (
     "vsprintf" => 1,
     "strcat" => 1,
     "strncat" => 1,
+    "strncpy" => 1,
+    "strtok_r" => 1,
+    "strtoul" => 1,
     "_mbscat" => 1,
     "_mbsncat" => 1,
     "_tcscat" => 1,