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/testusers.c

@@ -33,9 +33,9 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include <users.h>
+#include <maxscale/users.h>
 
-#include "log_manager.h"
+#include <maxscale/log_manager.h>
 
 /**
  * test1    Allocate table of users and mess around with it