MXS-2574: Add PATCH for /users/inet endpoint

The alteration of user passwords is now done inside MaxScale. This
prevents the possibility of a user locking themselves out.

include/maxscale/users.h

@@ -97,6 +97,17 @@ bool users_auth(USERS* users, const char* user, const char* password);
  */
 bool users_find(USERS* users, const char* user);
 
+/**
+ * Change password for a user
+ *
+ * @param users    The users table
+ * @param user     User to alter
+ * @param password The new password for the user
+ *
+ * @return True if password was changed
+ */
+bool users_change_password(USERS* users, const char* user, const char* password);
+
 /**
  * Check if user is an administrator
  *