Cleanup header files

- All now include maxscale/cdefs.h as the very first file.
- MXS_[BEGIN|END]_DECLS added to all C-headers.
  Strictly speaking not necessary for private headers, but
  does not hurt either.
- Include guards moved to the very top of the file.
- #pragma once added.

include/maxscale/listener.h

@@ -1,5 +1,6 @@
-#ifndef _LISTENER_H
-#define _LISTENER_H
+#pragma once
+#ifndef _MAXSCALE_LISTENER_H
+#define _MAXSCALE_LISTENER_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -27,10 +28,13 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/gw_protocol.h>
 #include <maxscale/gw_ssl.h>
 #include <maxscale/hashtable.h>
 
+MXS_BEGIN_DECLS
+
 struct dcb;
 struct service;
 
@@ -65,4 +69,6 @@ int listener_set_ssl_version(SSL_LISTENER *ssl_listener, char* version);
 void listener_set_certificates(SSL_LISTENER *ssl_listener, char* cert, char* key, char* ca_cert);
 int listener_init_SSL(SSL_LISTENER *ssl_listener);
 
+MXS_END_DECLS
+
 #endif