Adjust pty and tty event filters

Adjust pty and tty event filters

* Implement EVFILT_EXCEPT for ttys for HUP condition detection.
  This filter is used when pollfd.events has no read/write events.

* Add HUP condition detection to filt_ptcwrite() and filt_ttywrite()
  to reflect ptcpoll() and ttpoll(). Only poll(2) and select(2) can
  utilize the code; kevent(2) should behave as before with EVFILT_WRITE.

* Clear EV_EOF and __EV_HUP if the EOF/HUP condition ends.

OK mpi@