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
