Added check for server port range from environment variable

In my use case, I have several servers behind the same NAT firewall, and it becomes difficult to remember on the client side what is the correct UDP port range forwarded to each server.  So I added a simple check in mosh-server for the MOSH_SERVER_PORT environment variable and I set the correct port range in my authorized_keys file.  Now, regardless of where I connect from, my mosh client is given an appropriate UDP port.