Pass raw password to users_auth

By passing the raw password deeper into the authentication code, it can be
used to verify the user can access some systems. Right now, this is not
required by the simple salted password comparison done in MaxScale.

include/maxscale/users.h

@@ -100,12 +100,13 @@ bool users_find(USERS* users, const char* user);
 /**
  * Check if user is an administrator
  *
- * @param users The users table
- * @param user  User to check
+ * @param users    The users table
+ * @param user     User to check
+ * @param password Password of the user or NULL if password isn't available
  *
  * @return True if user is an administrator
  */
-bool users_is_admin(USERS* users, const char* user);
+bool users_is_admin(USERS* users, const char* user, const char* password);
 
 /**
  * Check how many admin account exists