don’t use reversed identifiers for include guards (Thanks Markus)

Done with:

    sed -in 's/\(ifndef\|define\) _\([0-9A-Z_]*\)$/\1 I3_\2/' include/**/*.h

fixes #804

include/key_press.h

@@ -7,8 +7,8 @@
  * key_press.c: key press handler
  *
  */
-#ifndef _KEY_PRESS_H
-#define _KEY_PRESS_H
+#ifndef I3_KEY_PRESS_H
+#define I3_KEY_PRESS_H
 
 /**
  * There was a key press. We compare this key code with our bindings table and pass