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/display_version.h

@@ -7,8 +7,8 @@
  * display_version.c: displays the running i3 version, runs as part of
  *                    i3 --moreversion.
  */
-#ifndef _DISPLAY_VERSION_H
-#define _DISPLAY_VERSION_H
+#ifndef I3_DISPLAY_VERSION_H
+#define I3_DISPLAY_VERSION_H
 
 /**
  * Connects to i3 to find out the currently running version. Useful since it