New upstream release
    Do the right thing (tm) and revert to $upstream-behaviour:
    No longer install tmux setgid and no longer use /var/run/tmux
    for sockets. Use "tmux -S /var/run/tmux/tmux-`id -u`/default attach"
    if you need to access an "old" tmux session
    tmux can be used as a login shell so add it to /etc/shells