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/formdata.c

@@ -31,7 +31,7 @@ struct Curl_easy;
 
 #include "urldata.h" /* for struct Curl_easy */
 #include "mime.h"
-#include "strdup.h"
+#include "curlx/strdup.h"
 #include "bufref.h"
 #include "curlx/fopen.h"
 
@@ -696,7 +696,7 @@ static CURLcode setname(curl_mimepart *part, const char *name, size_t len)
 
   if(!name || !len)
     return curl_mime_name(part, name);
-  zname = Curl_memdup0(name, len);
+  zname = curlx_memdup0(name, len);
   if(!zname)
     return CURLE_OUT_OF_MEMORY;
   res = curl_mime_name(part, zname);