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

src/tool_setup.h

@@ -62,12 +62,6 @@ extern FILE *tool_stderr;
 /* define what to use for unprintable characters */
 #define UNPRINTABLE_CHAR '.'
 
-#ifndef HAVE_STRDUP
-#include "tool_strdup.h"
-#undef Curl_strdup
-#define Curl_strdup tool_strdup
-#endif
-
 #ifndef tool_nop_stmt
 #define tool_nop_stmt do {} while(0)
 #endif