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/mime.c

@@ -30,7 +30,7 @@ struct Curl_easy;
 #include "sendf.h"
 #include "curl_trc.h"
 #include "transfer.h"
-#include "strdup.h"
+#include "curlx/strdup.h"
 #include "curlx/basename.h"
 #include "curlx/strcopy.h"
 #include "curlx/fopen.h"
@@ -1281,7 +1281,7 @@ CURLcode curl_mime_data(curl_mimepart *part, const char *ptr, size_t datasize)
     if(datasize == CURL_ZERO_TERMINATED)
       datasize = strlen(ptr);
 
-    part->data = Curl_memdup0(ptr, datasize);
+    part->data = curlx_memdup0(ptr, datasize);
     if(!part->data)
       return CURLE_OUT_OF_MEMORY;