Put tmux in extra-software

[bbaren]

A user wanted it, so we should at least consider adding it.

[geofft]

See MetapackagePolicy#debathena-extra-software. I would ask if it's fine to put it in a locker -- anyone know how it deals with sockets and server processes, and whether it needs privileged configuration on the local machine in order to work? If it does, I actually think that's a compelling reason to put it in the release.

(One could also argue that we need it to avoid taking sides in the religious debate with screen, but that's a silly argument.)

[kaduk]

I did build tmux in the bsd locker at some point, though I haven't been keeping it particularly updated.
Maybe Ben will do better at keeping it updated than I have been.