Allow choice of login method

[Zisu Andrei]

At the moment PAM modules can only alternate sequentially. For example, you cannot choose between fingerprint/password/face-recognition login. You have to wait for fingerprint verification to expire to move on to password login and so on.

This text is taken from Ubuntu fprintd documentation:

> At the moment, you cannot type in your password right away when you are asked for fingerprint. You need to make the fingerprint authentication fail first (swipe wrong finger or let it time out) before you are asked for password. This is a limitation of PAM because its modules mustn't be threaded and hence cannot support multiple means of authentication at the same time. (The old ThinkFinger used to do this, but it was a gross hack which caused many troubles.) A possible solution to this limitation is to make gdm, screensaver and policykit-1 support multiple alternative PAM stacks. Fedora 12 has enhanced the GDM login screen with a button that switches between password and fingerprint authentication mode (screenshot: http://bit.ly/cEjYbo). Similar enhancements for screensaver and PolicyKit are being worked on by Red Hat (see https://bugzilla.redhat.com/show_bug.cgi?id=500338).

A bug also needs to be opened upstream in lightdm for this, but for now I'm keeping track of it here.