Global timeout 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.
Functuonality 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.

(adaptation)