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/tree.h

@@ -7,8 +7,8 @@
  * tree.c: Everything that primarily modifies the layout tree data structure.
  *
  */
-#ifndef _TREE_H
-#define _TREE_H
+#ifndef I3_TREE_H
+#define I3_TREE_H
 
 extern Con *croot;
 /* TODO: i am not sure yet how much access to the focused container should