DialContext limited only the TCP establishment, so a peer that accepted
the connection and then stayed silent left gossh.NewClientConn blocking
forever and the terminal stuck on "Connecting".
Set the socket deadline from the dial context before the handshake and
clear it on success, so the handshake shares the dial timeout instead of
being able to hang. Verified against a silent listener: the connect now
returns i/o timeout instead of blocking.
cc0702396c