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/core/test/testadminusers.c

@@ -34,8 +34,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <gwdirs.h>
-#include <adminusers.h>
+#include <maxscale/gwdirs.h>
+#include <maxscale/adminusers.h>
 #include <maxscale/alloc.h>