build: tidy up and dedupe strdup functions

- 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.

Closes #20497

tests/server/.checksrc

@@ -16,5 +16,4 @@ allowfunc send
 allowfunc snprintf
 allowfunc socket
 allowfunc sscanf
-allowfunc strdup
 allowfunc vsnprintf

tests/server/first.h

@@ -69,10 +69,6 @@ extern const struct entry_s s_entries[];
 #  define snprintf _snprintf
 #endif
 
-#ifdef _WIN32
-#  define strdup _strdup
-#endif
-
 #ifdef _WIN32
 #  define CURL_STRNICMP(p1, p2, n) _strnicmp(p1, p2, n)
 #elif defined(HAVE_STRCASECMP)

tests/server/tftpd.c

@@ -610,7 +610,7 @@ static int validate_access(struct testcase *test,
                             (long)our_getpid());
 
     logmsg("Are-we-friendly question received");
-    test->buffer = strdup(weare);
+    test->buffer = curlx_strdup(weare);
     test->rptr = test->buffer; /* set read pointer */
     test->bufsize = count;    /* set total count */
     test->rcount = count;     /* set data left to read */