Ensure sorting of admin forms uses alphabetical sorting when role weights are equal

With the current dev branch with this #210457 applied, sorting is less than satisfactory when using 'role_weights_reorder_forms' = TRUE and role weights are equal. We should fall back to alphabetical in this case.

This is marked minor as the $conf setting is hidden from most users (no UI), and I'm guessing that if this variable is set then the site admin will also be setting different weights... occasionally however a site may require that two roles have the same weight.