memory: make function overrides work reliably in unity builds

[vszakats]

Fixing:

• HTTPS-RR builds with c-ares and Linux MUSL.
• curl-for-win minimal builds with Linux MUSL.

It should fix all other kinds of entaglement between curl's redefintions
of system symbols and system (or 3rd-party) headers sensitive to that.

It also syncs memory override behavior between unity & non-unity builds,
thus reducing build variations.

The idea is to define and declare everything once in curl_setup.h,
without overriding any system symbols with curl ones yet. Then, like
before this patch, override them, if necessary, in each source file via
curl_memory.h and memdebug.h, after including system headers.
To ensure a clean slate with no overrides at the beginning of each
source file, reset all of them unconditionally at the end of
curl_setup.h, by including curl_mem_undef.h. (This assumes
curl_setup.h is always included first, which is already the case
throughout the codebase.)

curl_mem_undef.h can also be included explicitly wherever overrides
are causing problems. E.g. in tests which use unity-style builds and
a previously included curl_memory.h/memdebug.h can be spilling into
other source files.

The simplified role of the two override headers:

• curl_memory.h: overrides system memory allocator functions to
  libcurl ones, when memory tracing (aka CURLDEBUG) is disabled.
• memdebug.h: overrides system memory allocator and some other
  functions to curl debug functions, when memory tracing is enabled.

Changed made in this patch, step-by-step:

• curl_memory.h: move allocator typedefs and protos to curl_setup.h.
• memdebug.h: move ALLOC_* macros to curl_setup.h.
• memdebug.h: move allocator protos to curl_setup.h.
• memdebug.h: move Curl_safefree() macro to curl_setup.h.
  (it's a regular macro, with a one-time, global, definition.)
• curl_memory.h: move system symbol undefs to a new, separate header:
  curl_mem_undef.h.
• curl_setup.h: include curl_mem_undef.h at the end, unconditionally,
  to reset system symbol macros after each inclusion.
• handle sclose() and fake_sclose() in curl_setup.h. They are not
  system symbols, a one-time definition does the job.

Also:

• GHA/linux: enable unity mode for the HTTP-RR c-ares MUSL job.
  Follow-up to 17ab4d6 GHA: add a build with c-ares + HTTP-RR on Alpine #16413

That said, I'd still find it better to avoid redefining system macros.
To communicate clearly the fact that they are not the original system
calls and they do behave differently. And, it would allow dropping the
undef/redef dance in each source file, and maintaining the logic with
it. The "last #include files should be in this order" comments in each
source would also become unnecessary. Also the trick of using
(func) (or interim macros) to call the non-overridden function where
required. This method works for printf and most everything else already.
For _tcsdup, socket and fopen functions this could work without
disturbing the codebase much.

Ref: #16428 (clean reboot of)

---

• tidy up the way fake_sclose() is defined. move non-debug define to curl_memory.h, or maybe move the logic to cfilters.c, its only user?
• drop extra non-debug Alpine job, used for testing here.
• move unrelated CURL_EXTERN cleanup to separate PR → ws: drop redundant CURL_EXTERN from function definitions #17832
• move unrelated CURL_MT_LOGFNAME_BUFSIZE tidy-up to separate PR. → memdebug.h: eliminate global macro CURL_MT_LOGFNAME_BUFSIZE #17833
• fix to undef accept4 → curl_memory.h: fix to undefine accept4 #17831
• fix newly revealed valgrind issue for test 4001 → rustls: apply memory function overrides, fixing an ECH buffer free #17830
  https://github.com/curl/curl/actions/runs/16093372427/job/45421778472?pr=17827#step:39:3321
• rebase on top of split off PRs.
• drop the undef/redef dance for macro that ARE NOT system symbols: sclose and fake_sclose.
• rebase on: drop unused wcsdup/_wcsdup overrides → memory: stop overriding unused wcsdup()/_wcsdup() system functions #17840

[vszakats]

@bagder: This PR may also be necessary to avoid system symbol override issues in the handful (my manual count) of tests that include system headers.