build: drop duplicate C includes

- `stdlib.h` and `string.h` is included via `curl_setup_once.h`,
  allowing to drop from `tests/server` sources and `tool_doswin.c`.

- `stdlib.h` is also included via `setup-vms.h` (earlier than above),
  allowing to drop it from `curl_addrinfo.h` on VMS.

Closes #20303

lib/curl_addrinfo.h

@@ -38,7 +38,6 @@
 #ifdef __VMS
 #  include <in.h>
 #  include <inet.h>
-#  include <stdlib.h>
 #endif
 
 /*