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
 
 /*

src/tool_doswin.c

@@ -30,7 +30,6 @@
 #endif
 
 #ifdef _WIN32
-#  include <stdlib.h>
 #  include <tlhelp32.h>
 #endif
 

tests/server/first.c

@@ -24,7 +24,6 @@
 #include "first.h"
 
 #include <stdio.h>
-#include <string.h>
 
 int main(int argc, char **argv)
 {

tests/server/mqttd.c

@@ -23,9 +23,6 @@
  ***************************************************************************/
 #include "first.h"
 
-#include <stdlib.h>
-#include <string.h>
-
 /* Function
  *
  * Accepts a TCP connection on a custom port (IPv4 or IPv6).  Speaks MQTT.

tests/server/socksd.c

@@ -23,8 +23,6 @@
  ***************************************************************************/
 #include "first.h"
 
-#include <stdlib.h>
-
 /* Function
  *
  * Accepts a TCP connection on a custom port (IPv4 or IPv6). Connects to a