mosh: Support NAT firewall port re-writing

https://github.com/keithw/mosh/issues/494

Some firewalls re-write port numbers. Allow a port translation rule
to be specified. When the mosh-server responds with a port, have
the mosh-client connect to the translated external firewall port.

Signed-off-by: Earl Chew <earl_chew@yahoo.com>