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/dcb.h

@@ -1,5 +1,6 @@
-#ifndef _DCB_H
-#define _DCB_H
+#pragma once
+#ifndef _MAXSCALE_DCB_H
+#define _MAXSCALE_DCB_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -12,6 +13,7 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+#include <maxscale/cdefs.h>
 #include <maxscale/spinlock.h>
 #include <maxscale/buffer.h>
 #include <maxscale/listmanager.h>
@@ -23,6 +25,8 @@
 #include <maxscale/skygw_utils.h>
 #include <netinet/in.h>
 
+MXS_BEGIN_DECLS
+
 #define ERRHANDLE
 
 struct session;
@@ -369,4 +373,7 @@ void dcb_append_readqueue(DCB *dcb, GWBUF *buffer);
 
 #define DCB_IS_CLONE(d) ((d)->flags & DCBF_CLONE)
 #define DCB_REPLIED(d) ((d)->flags & DCBF_REPLIED)
+
+MXS_END_DECLS
+
 #endif /*  _DCB_H */