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

REUSE.toml

@@ -38,17 +38,15 @@ path = [
   "renovate.json",
   "tests/certs/**",
   "tests/data/test**",
-  "tests/stunnel.pem",
   "tests/valgrind.supp",
   # checksrc control files
-  "docs/examples/.checksrc",
   "lib/.checksrc",
+  "lib/curlx/.checksrc",
   "lib/vauth/.checksrc",
   "lib/vquic/.checksrc",
   "lib/vssh/.checksrc",
   "lib/vtls/.checksrc",
   "src/.checksrc",
-  "tests/libtest/.checksrc",
   "tests/server/.checksrc",
 ]
 SPDX-FileCopyrightText = "Daniel Stenberg, <daniel@haxx.se>, et al."