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/startup.h

@@ -10,8 +10,8 @@
  *            the appropriate workspace.
  *
  */
-#ifndef _STARTUP_H
-#define _STARTUP_H
+#ifndef I3_STARTUP_H
+#define I3_STARTUP_H
 
 #define SN_API_NOT_YET_FROZEN 1
 #include <libsn/sn-monitor.h>