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.

server/modules/protocol/maxscaled/maxscaled.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MAXSCALED_H
 #define _MAXSCALED_H
 /*
@@ -27,6 +28,9 @@
 #include <maxscale/dcb.h>
 #include <maxscale/spinlock.h>
 #include <maxscale/housekeeper.h>
+
+MXS_BEGIN_DECLS
+
 /**
  * The maxscaled specific protocol structure to put in the DCB.
  */
@@ -41,4 +45,6 @@ typedef struct maxscaled
 #define MAXSCALED_STATE_PASSWD  2   /**< Waiting for password */
 #define MAXSCALED_STATE_DATA    3   /**< User logged in */
 
+MXS_END_DECLS
+
 #endif