Persisting/restoring ssh-agent across screen/tmux sessions  

As a heavy user of screen and now of tmux, I find it annoying that each time I disconnect the terminal and reconnect and then reattach to screen/tmux, I find that inside the screen/tmux sessions, SSH_AUTH_SOCK variable is stale: it does not point anymore to the ‘current’ file. So, here is a script to fix that: