use #pragma once instead of foo*foo #define guards

#pragma once has been "un-deprecated" in gcc since 3.3, and is widely supported
in other compilers.

I've been using and maintaining (rebasing) this patch for a while now, as
it annoyed me to see #ifndef fooblahfoo, etc all over the place,
almost arrogant about the annoyance of having to define all these names to
perform a commen but neccicary functionality, when a completely superior
alternative exists.

I havn't sent it till now, cause its kindof a style change, and it is bad
voodoo to mess with style that has been established by more established
editors. So feel free to lambast me as a crazy bafoon.

v2 - preserve externally used headers

src/core/loopback-setup.h

@@ -1,7 +1,6 @@
 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
 
-#ifndef fooloopbacksetuphfoo
-#define fooloopbacksetuphfoo
+#pragma once
 
 /***
   This file is part of systemd.
@@ -23,5 +22,3 @@
 ***/
 
 int loopback_setup(void);
-
-#endif