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/manage.h

@@ -7,8 +7,8 @@
  * manage.c: Initially managing new windows (or existing ones on restart).
  *
  */
-#ifndef _MANAGE_H
-#define _MANAGE_H
+#ifndef I3_MANAGE_H
+#define I3_MANAGE_H
 
 #include "data.h"