tree-wide: Clean up includes

This commit cleans up the includes for all the small tools across
the tree.

A few cases of returning EXIT_SUCCESS are replaced with returning
0 to avoid including <stdlib.h>.

Split out of #37344.

src/ssh-generator/ssh-proxy.c

@@ -1,7 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <net/if_arp.h>
-#include <stdio.h>
 #include <unistd.h>
 
 #include "sd-varlink.h"
@@ -13,7 +12,6 @@
 #include "log.h"
 #include "main-func.h"
 #include "missing_socket.h"
-#include "parse-util.h"
 #include "socket-util.h"
 #include "string-util.h"
 #include "strv.h"