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/adminusers.c

@@ -19,11 +19,11 @@
 #endif
 #include <unistd.h>
 #include <crypt.h>
-#include <users.h>
-#include <adminusers.h>
-#include <skygw_utils.h>
-#include <log_manager.h>
-#include <gwdirs.h>
+#include <maxscale/users.h>
+#include <maxscale/adminusers.h>
+#include <maxscale/skygw_utils.h>
+#include <maxscale/log_manager.h>
+#include <maxscale/gwdirs.h>
 #include <sys/stat.h>
 
 /**