Provide an alternative mechanism for extracting the server IP address

[pcc]

On some installations of ssh, overriding ProxyCommand is forbidden,
which means that the fake proxy won't work. Instead, have mosh-server
report the local address in SSH_CONNECTION, which will be used if
the fake proxy doesn't report an address.

[andersk]

This will result in a MOSH SSHIP line spewed to the console when an old client connects to a new server.

(It might be wise for us to develop a framework for adding extensions to the startup protocol without this kind of breakage.)

[cgull]

There's a somewhat hackish way around this: mosh could do ssh $host 'echo MOSH SSHIP SSH_CONNECTION; mosh ...'