loginDialog: ask for username up front if disable-user-list==TRUE

Right now, we rely on PAM to ask for the username if disable-user-list
is TRUE.  This is suboptimal because it means we can't check if we
should show a session menu.

This commit changes disable-user-list==TRUE to ask for a username up
front, rather than have PAM do it.

https://bugzilla.gnome.org/show_bug.cgi?id=706607
src: Eliminate compiler warnings