build: tidy up and dedupe strdup functions

[vszakats]

• de-dupe lib/src strdup/memdup functions into curlx.
• introduce CURLX_STRDUP_LOW() for mapping strdup(), and to do it at
  one place within the code, in curl_setup.h.
• tests/server: use curlx_strdup(). (Also to fix building without
  a system strdup().)
• curlx/curlx.h: shorten and tidy up.
• adjust Windows build path to not need HAVE_STRDUP.
• build: stop detecting HAVE_STRDUP on Windows.

---

• it looks like Curl_strdup() isn't used except for one place. Make makes me doubt curl builds without HAVE_STRDUP. tests/server uses strdup without checking HAVE_STRDUP.
  My assessment is that this was the case for a long time, but only became apparent via a bare grep after 193cb00 build: stop overriding standard memory allocation functions #19626.
• test the above more. possibly it applied to tests only, and certainly to examples.
  edit: confirmed that in 8.17 curl and libcurl built without strdup, but tests and examples did not.
• move dropping HAVE_STRDUP to deparate PR.
• rename functions moved to curlx to have curlx_ prefixes.
• classic mac may be a platform without strdup (deducting from config-mac.h, which missed setting HAVE_STRDUP). 919c97f. edit: classic mac includes extra/strdup.h, so it also has one.

Separate PRs:

• drop unused Curl_saferealloc() function.
  Unused since 67ae101 http: unfold response headers earlier #19949 → curlx: drop unused curlx_saferealloc() #20504
• build: assume strdup() C function, drop feature detection.
  curl did not build without a system strdup() for probably a good
  while. This became apparent (with a grep) after
  193cb00 build: stop overriding standard memory allocation functions #19626.
  Let us know if this causes an issue. → build: require POSIX strdup() #20505