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/sysctl/sysctl.c

@@ -1,13 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include <errno.h>
 #include <getopt.h>
-#include <limits.h>
-#include <stdbool.h>
 #include <stdio.h>
-#include <stdlib.h>
 #include <sys/stat.h>
-#include <sys/types.h>
 
 #include "alloc-util.h"
 #include "build.h"