Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

server/modules/include/maxscaled.h

@@ -24,9 +24,9 @@
  *
  * @endverbatim
  */
-#include <dcb.h>
-#include <spinlock.h>
-#include <housekeeper.h>
+#include <maxscale/dcb.h>
+#include <maxscale/spinlock.h>
+#include <maxscale/housekeeper.h>
 /**
  * The maxscaled specific protocol structure to put in the DCB.
  */