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

lib/setopt.c

@@ -46,7 +46,7 @@
 #include "altsvc.h"
 #include "hsts.h"
 #include "tftp.h"
-#include "strdup.h"
+#include "curlx/strdup.h"
 #include "escape.h"
 #include "bufref.h"
 
@@ -1920,7 +1920,7 @@ static CURLcode setopt_cptr(struct Curl_easy *data, CURLoption option,
            mark that postfields is used rather than read function or form
            data.
         */
-        char *p = Curl_memdup0(ptr, pflen);
+        char *p = curlx_memdup0(ptr, pflen);
         if(!p)
           return CURLE_OUT_OF_MEMORY;
         else {