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/load_layout.h

@@ -8,8 +8,8 @@
  *                restart.
  *
  */
-#ifndef _LOAD_LAYOUT_H
-#define _LOAD_LAYOUT_H
+#ifndef I3_LOAD_LAYOUT_H
+#define I3_LOAD_LAYOUT_H
 
 void tree_append_json(const char *filename);