It is common case when modem hangs with carier on but don't
receive anything from another side.
This thing commonly healed with hangup and redialing.
Enhancements below allows to determine when such action
is needed and inform attach program with SIGURG signal.
There two ioctls set: outfill and keepalive, used from both
sides of connection. Outfill repeatedly sends FRAME_END with
specified timeout (i.e. 40 seconds). It is needed to get input on
other side even if no user activity on slip line currently.
Keepalive checks FRAME_ENDs from other side, and if no one
got in specified timeout (i.e. 60 seconds, max modem retrain time),
send SIGURG to attach program.
I plan to add code to slattach to handle this thing too.
Reviewed by: wollman
ca9301398c