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_getparam.c

@@ -36,7 +36,6 @@
 #include "tool_main.h"
 #include "tool_stderr.h"
 #include "tool_help.h"
-#include "tool_strdup.h"
 #include "var.h"
 
 #define ALLOW_BLANK TRUE
@@ -70,7 +69,7 @@ static ParameterError getstrn(char **str, const char *val,
   if(!allowblank && !val[0])
     return PARAM_BLANK_STRING;
 
-  *str = memdup0(val, len);
+  *str = curlx_memdup0(val, len);
   if(!*str)
     return PARAM_NO_MEM;