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/xmlstream.c

@@ -122,7 +122,7 @@ int main(void)
   struct ParserStruct state;
 
   /* Initialize the state structure for parsing. */
-  memset(&state, 0, sizeof(struct ParserStruct));
+  memset(&state, 0, sizeof(state));
   state.ok = 1;
 
   /* Initialize a namespace-aware parser. */