session: Only emit verification-complete on reauth or after session is opened

We're currently emitting verification-complete signal as soon as we got a positive authentication callback from the worker, however at this point we've not opened the session yet, and if that fails we'll end up in a scenario where gdm assumed that we're logging in, and so its greeter is shut down and never restarted on its VT until we manually switch back and forth from it. Meanwhile the session opening failure is never exposed to the user.

So, do not emit verification-completed on logins, until we've opened the session. However, re-order the session-opened callback events so that we keep notifying verification-complete before than session-opened.

This basically reverts commit c4292870 (again).