support ssh multiplexer in tunnels #4717

commit 3967cb5a86e0a2206f435fb740ef9c354eef8edd forced multiplexing off with "-S none", overriding the user's ssh configuration, to work around a bug in openssh (#4717, #4720)

This patch uses the multiplex connection if one is available, while avoiding the problem with the "-f" bug.

Reason: our university's cluster blocks you for half an hour if you make more than 3 connections to 
port 22 within one minute. This makes IPython unusable via ssh at the moment. I think this patch resolves it. 
Have done some minimal testing, with/without mux and with/without public key auth.