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.

src/xcursor.c

@@ -1,5 +1,3 @@
-#undef I3__FILE__
-#define I3__FILE__ "xcursor.c"
 /*
  * vim:ts=4:sw=4:expandtab
  *
@@ -9,6 +7,8 @@
  * xcursor.c: xcursor support for themed cursors.
  *
  */
+#include <config.h>
+
 #include <assert.h>
 #include <xcb/xcb_cursor.h>