build: avoid overriding system symbols for socket functions

Before this patch `accept4()`, `socket()`, `socketpair()`, `send()` and
`recv()` system symbols were remapped via macros, using the same name,
to local curl debug wrappers. This patch replaces these overrides by
introducing curl-namespaced macros that map either to the system symbols
or to their curl debug wrappers in `CURLDEBUG` (TrackMemory) builds.

This follows a patch that implemented the same for `accept()`.

The old method required tricks to make these redefines work in unity
builds, and avoid them interfering with system headers. These tricks
did not work for system symbols implemented as macros.

The new method allows to setup these mappings once, without interfering
with system headers, upstream macros, or unity builds. It makes builds
more robust.

Also:
- checksrc: ban all mapped functions.
- docs/examples: tidy up checksrc rules.

Follow-up to 9863599d69 #18502
Follow-up to 3bb5e58c10 #17827

Closes #18503

REUSE.toml

@@ -40,6 +40,7 @@ path = [
   "tests/data/test**",
   "tests/valgrind.supp",
   # checksrc control files
+  "docs/examples/.checksrc",
   "lib/.checksrc",
   "lib/curlx/.checksrc",
   "lib/vauth/.checksrc",