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/session.h

@@ -1,5 +1,6 @@
-#ifndef _SESSION_H
-#define _SESSION_H
+#pragma once
+#ifndef _MAXSCALE_SESSION_H
+#define _MAXSCALE_SESSION_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -33,6 +34,8 @@
  *
  * @endverbatim
  */
+
+#include <maxscale/cdefs.h>
 #include <time.h>
 #include <maxscale/atomic.h>
 #include <maxscale/buffer.h>
@@ -42,6 +45,8 @@
 #include <maxscale/skygw_utils.h>
 #include <maxscale/log_manager.h>
 
+MXS_BEGIN_DECLS
+
 struct dcb;
 struct service;
 struct filter_def;
@@ -201,4 +206,7 @@ void session_disable_log_priority(SESSION* ses, int priority);
 RESULTSET *sessionGetList(SESSIONLISTFILTER);
 void process_idle_sessions();
 void enable_session_timeouts();
+
+MXS_END_DECLS
+
 #endif