Code review comment for lp:~mterry/lightdm/no-dmrc

I think we need to continue to support .dmrc for systems that don't have accounts service:
http://lists.freedesktop.org/archives/lightdm/2011-August/000051.html

What we should do is merge user.c into the new accounts.c so that internally LightDM uses the accounts service interface but the backend can fallback to passwd and .dmrc.

Also since it doesn't make sense to have multiple accounts instances inside LightDM I'd make it a singleton and just access through functions for simplicity.

In terms of Unity/GNOME, I definitely think we should be using accounts service.