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/thread.h

@@ -1,5 +1,6 @@
-#ifndef _THREAD_H
-#define _THREAD_H
+#pragma once
+#ifndef _MAXSCALE_THREAD_H
+#define _MAXSCALE_THREAD_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -22,6 +23,10 @@
  * of changes.
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 /**
  * Thread type and thread identifier function macros
  */
@@ -33,4 +38,6 @@ extern THREAD *thread_start(THREAD *thd, void (*entry)(void *), void *arg);
 extern void thread_wait(THREAD thd);
 extern void thread_millisleep(int ms);
 
+MXS_END_DECLS
+
 #endif