Bug#829591: hurd: pseudoterminal hangs when I press ^O (flush)

[Kalle Olavi Niemitalo <kon@iki.fi>]

Control: retitle -1 term should clear discard-output flag in more situations

Although the FreeBSD manual page for termios(4)
https://www.freebsd.org/cgi/man.cgi?query=termios&apropos=0&sektion=4&manpath=FreeBSD+10.3-RELEASE+and+Ports&arch=default&format=html
documents the DISCARD key, the kernel doesn't appear to implement it:
https://svnweb.freebsd.org/base/stable/10/sys/kern/tty_ttydisc.c?revision=256281&view=markup#l897
There, the ttydisc_rint function recognizes VLNEXT but not VDISCARD.

In contrast, NetBSD implements DISCARD:
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/kern/tty.c?rev=1.271&content-type=text/x-cvsweb-markup&only_with_tag=MAIN
There, the ttyinput_wlock function recognizes VDISCARD and toggles
the FLUSHO flag.  This flag is also cleared in several more places:
* At the end of ttyinput_wlock, except in these cases:
  - a break, parity error, or a framing error was ignored
  - the DISCARD or STOP character was just received
  - output remains suspended with STOP.
* In the ttioctl function, if the TIOCSTART ioctl is used.
* In the ttioctl function, if the TIOCSETA, TIOCSETAW, or
  TIOCSETAF ioctl is used.  It actually copies the flag from
  the FLUSHO bit of termios::c_lflag.
* In the ttyrub function, if the ECHO flag is set and the EXTPROC
  flag is not set.  This function is called when a previously
  typed character needs to be deleted.
* In the ttyecho function, unless it's being called because the
  user pressed the TAB key.

The term translator should thus be changed:
* Clear the discard-output flag when the user types almost any
  other character, like the glibc documentation said.
* Clear the discard-output flag when TIOCSTART is used.
  This is term/users.c (S_tioctl_tiocstart), I believe.
* Copy the discard-output flag from the FLUSHO bit of
  termios::c_lflag, when TIOCSETA, TIOCSETAW, or TIOCSETAF is
  used.  This is term/users.c (set_state), I believe.  The GNU C
  Library already defines FLUSHO in both <bits/ioctls.h> and
  <bits/termios.h>.

With those changes, I think my previous open_hook patch would
no longer be necessary because sshd would call tcsetattr without
FLUSHO and thereby clear the flag.