Ensure all *.[ch] files include config.h

Including config.h is necessary to get e.g. the _GNU_SOURCE define and
any other definitions that autoconf declares. Hence, config.h needs to
be included as the first header in each file.

This is done either via:
1. Including "common.h" (i3bar)
2. Including "libi3.h"
3. Including "all.h" (i3)
4. Including <config.h> directly

Also remove now-unused I3__FILE__, add copyright/license statement
where missing and switch include/all.h to #pragma once.

i3-msg/main.c

@@ -14,6 +14,8 @@
  * Additionally, it’s even useful sometimes :-).
  *
  */
+#include "libi3.h"
+
 #include <stdio.h>
 #include <stdbool.h>
 #include <sys/types.h>
@@ -34,7 +36,6 @@
 #include <xcb/xcb.h>
 #include <xcb/xcb_aux.h>
 
-#include "libi3.h"
 #include <i3/ipc.h>
 
 static char *socket_path;