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.

libi3/get_exe_path.c

@@ -1,3 +1,12 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ */
+#include "libi3.h"
+
 #include <unistd.h>
 #include <string.h>
 #include <stdio.h>
@@ -5,8 +14,6 @@
 #include <stdlib.h>
 #include <errno.h>
 
-#include "libi3.h"
-
 /*
  * This function returns the absolute path to the executable it is running in.
  *