checksrc: reduce exceptions, apply again to curlx

[vszakats]

• 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.

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• examples: drop typedef struct
• move some banned funcs to the default checksrc list?