Support port range as -p/--port PORT[:HIGHPORT].

Extend mosh and mosh-server to support parsing a high port from
the desired_port argument.  The first (low) port must not be
greater than the second (high) port.
If only one value is provided, behaviour is as before; bind
to one port.  Also tweak the formatting in mosh-server(1) synopsis
to be consistent.

This resolves mosh issue # 296.

Signed-off-by: Luke Mewburn <luke@mewburn.net>