Fix races between tty.c and sessrele() / doenterpgrp() / leavepgrp(). The tty
code is still under giant lock, but the session/pgrp release code just used
proctree_locks. This explains why moving the proctree_lock in sys/kern/tty.c
rev. 1.258 did fix the panics in our SMP systems.

This should also fix some race panics with revoked ttys.

Reviewed by: jhb
MFC after: 1 week