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/render.h

@@ -8,8 +8,8 @@
  *           various rects. Needs to be pushed to X11 (see x.c) to be visible.
  *
  */
-#ifndef _RENDER_H
-#define _RENDER_H
+#ifndef I3_RENDER_H
+#define I3_RENDER_H
 
 /**
  * "Renders" the given container (and its children), meaning that all rects are