loginDialog: don't rely on PAM to ask for a username

For the "Not Listed?" case we will need to be able
to identify when the user has entered their username.

Once we have a way of tracking when the username is
entered, we can then defer showing the session list
too early, before the user can reliably pick a
session.

This username tracking will also be important for
implementing a disable-user-list configuration key.
If the config key gets toggled off at runtime, we'll
need to know if we're at a disruptive part of
the authentication process or not, so we know whether
we can can expose the user list right away, or wait
until the authentication conversation finishes.

Right now, we pass null in for an initial username,
and let the PAM machinery ask the user, which means we
have no good way of knowing when the username is entered.

This commit changes the "Not Listed?" code to ask the
user their username up front, before starting the PAM
conversation in much the same way we do if the user
picks a user from the user list.

https://bugzilla.gnome.org/show_bug.cgi?id=660660