Add support to limit the port range for dynamic binds

[stromnet]

This feature is required to work in an environment with at least somewhat sane firewall rules.

Usage:

./shairplay .... --dyn_port_min=5000 --dyn_port_max=5010

This will ensure that the UDP/TCP ports dynamically bound will be betewen 5000 and 5010 (inclusive). If not specified (or 0), default behaviour is used (random port as decided by the system).

Binding changes have not been tested (not even compiled).