There's a race condition on shutdown where the session worker is
switching VTs back to the initial VT at the same time as the session
exit is being processed.

This means that manager may try to start a login screen (because of
the VT switch) when autologin is enabled when there shouldn't be a
login screen.

This commit makes sure both the PostSession script, and session-exited
signal emission are complete before initiating the VT switch back
to the initial VT.

#660