I lose my agent forwarding

From time to time I lose my ssh-agent in a multiplexed ssh connection. Apparently, ssh is able to pick up the multiplexed connection, but is unable to reuse the ssh-agent socket. To fix this I need to kill the connection via ssh -O exit SERVER before reconnecting. I think this can cause quite some confusion for unexperienced users. So maybe we should set ControlPersist 20s (I think it needs to be > ServerAliveInterval) such that connections will be closed rather quickly if unused. You still have the benefit of reusing existing connections if you connect multiple times.

Ping @pibis what do you think?