allow client-side port or port range to be specified

It would be useful if mosh allowed the client port, or port range, to be specified. The envisaged scenario is something like:

1. set up firewall to pass through UDP port 60001
2. mosh -p 60001 -P 60001 server.example.com.

or with port ranges:

1. set up firewall to pass through UDP ports 6[01]000
2. mosh -p 60000:61000 -P 60000:61000

Mosh currently uses a system-assigned port number for mosh-client. It is therefore not possible to tunnel mosh through stateless firewalls, unless they are set up to allow essentially all UDP through.

(This scenario is common for DNS. For instance, a BIND name server can be configured to always use source port 53 when acting as a client, allowing a port 53-only UDP pass-through on the firewall.)

The code for server-side port ranges is already in place, via try_bind(), although this is not currently exposed to the user. What seems to be required is a modification to the client's Connect() to call try_bind() with a port or port range, if specified.

Start in the client's Connect() path and compare it with the existing server-side try_bind() support. Trace how client port options are parsed, then verify that a single port or range can be passed to mosh-client and used through a matching UDP firewall configuration. Done means both forms work without relying on a system-assigned client port.