add a "global" ChannelTimeout type to ssh(1) and sshd(8) that watches

add a "global" ChannelTimeout type to ssh(1) and sshd(8) that watches
all open channels and will close all open channels if there is no
traffic on any of them for the specified interval. This is in addition
to the existing per-channel timeouts added a few releases ago.

This supports use-cases like having a session + x11 forwarding channel
open where one may be idle for an extended period but the other is
actively used. The global timeout would allow closing both channels when
both have been idle for too long.

ok dtucker@