This ticket is to track implementation of login stacks.

Login stacks would allow multiple login stacks to be defined, and users get a selection on first entering /login/.

A login stack can contain various "intermediate" plugins, and one "final" plugin.
Intermediate would be for example gssapi/auth_kerb, where auth is performed automatically, invisible to the user.
A final plugin would be a module with a form.

Also, a login stack can contain up to 1 info plugin.