ssh/tmux: create consistent ssh-agent socket file

* "--quick" does not inherit agent from current ssh session
* update common socket when we rediscover agent
* generally set the AUTH to be the common socket, so that we update when
  we get a new connection
* ackrc - search "unknown" files too
* ssh-auth-sock - no circular references
* ssh-auth-sock - on /tmp to avoid NFS clashing
* tmux/ssh - actually set env var