slave: don't run initial-setup on remote displays

GDM will run initial-setup if there are no configured user accounts
on the system.  We correctly skip checking if there are configured user
accounts on remote XDMCP displays, but incorrectly, still run initial-setup.

This commit makes sure we only every do any of the initial-setup logic
in the local display code paths.

https://bugzilla.gnome.org/show_bug.cgi?id=711180
slave: don't kill slave when client disconnects

Right now we stop the slave if a greeter running on a remote
X server exits. This is wrong, since the greeter will exit as
part of the login screen process.  We already have mechanisms in
place to reap the slave at the appropriate time, so stopping the
slave on disconnection is totally unneeded and actively harmful.

This commit drops that code.

https://bugzilla.gnome.org/show_bug.cgi?id=690926
manager: return ACCESS DENIED for most open-reauth-channel failures

gnome-shell only falls back to opening a new session if the reauth
channel fails to open because of denied access.  For XDMCP we currently
fail for other reasons.

This commit returns ACCESS DENIED for those cases as well, so the
intended fallback happens.

Based on a patch by Jeremy White <jwhite@codeweavers.com>

https://bugzilla.gnome.org/show_bug.cgi?id=690926
daemon: don't require seat to locate reauth channel

With systemd, XDMCP sessions don't have associated seats.
Currently, reauth channels find the session by first looking up
the seat, which means unlocking fails for XDMCP.

This commit changes the code to determine session strictly from pid.

https://bugzilla.gnome.org/show_bug.cgi?id=690926
slave: Pass session id to unlocking code if we know it

We currently figure out which session to unlock on-the-fly based on
the username that was just reauthenticated.  This doesn't work, for
XDMCP sessions, under logind, since they don't have a seat. There's
no reason to try to deduce the session, though.  We already know it,
so we might as well pass it down.

This fixes unlocking for XDMCP.

https://bugzilla.gnome.org/show_bug.cgi?id=690926