kernel - More pty fixes

kernel - More pty fixes

* Allow the pts to be closed and reopened while the ptc is held open
  (xterm issues a revoke + open).  This fixes xterm.

* Add a flag to differentiate between a slave not yet initially opened
  and a slave which has been closed, so ptc reads and the kqfilter
  can properly return EOF (tmux).

* Remove pt_flags2 and fold its flags into pt_flags.

* Create a shim to track ttyunhold()'s for session releases and release
  the underlying pts (for screen).

Reported-by: josepht (xterm issue)