It's entirely possible for a session returned by
sd_seat_get_sessions to disappear immediately after the
sd_seat_get_sessions call returns.  This is especially
likely at logout time where the session will briefly be
in the "closing" state before getting reaped.

If that happens when we're looking for a greeter session, we
stop looking for a greeter session and bail out all confused.

This commit fixes the confusion by gracefully handling the
session disappearing by just proceeding to the next session
in the list.

This commit is very similar to commit 155ee7ec which got
accidentally reverted during code consolidation. The main
difference is this commit checks the correct error code
of -ENXIO instead of -ENOENT, so it might actually fix
what it's ostensibly supposed to fix.