Make UDP port configurable from environment

I have a use case where I can't use the default 60000-61000 port range. I want to configure the port range server-side as not to impose all users to set the port range manually.

This is a trivial change that makes the *default* value for -p be read from the environment **MOSH_SERVER_DEFAULT_PORT**. To use this, one can set that value in /etc/environment where available or use a wrapper something like

```bash
export MOSH_SERVER_DEFAULT_PORT=5500:6000
exec mosh "$@"
```