Secure Tmuxes on SIGHUP

[raymondberg]

This code improved the tmux sharing safety, but we really shouldn't rely on someone safely detaching or exiting their tmux in order to lock it down.

If the primary user leaves the session because their terminal hangs up it should do the same. You can achieve something like this with nohup and trap 'some cleanup code' HUP, but it's not easy in our current framework.