Add unused port helpers for UDP

[dbuse]

Extends the unused_tcp_port and unused_tcp_port_factory mechanisms for UDP ports.

[eirnym]

Could you make socket_type setting explicit for TCP as well? I understand that it's default, but it would be much easier to read

Also both factories unused_udp_port_factory and unused_tcp_port_factory are essencially the same. The only difference is socket_type.

Additionally, please, spread documentation to fixtures.

[dbuse]

Could you make socket_type setting explicit for TCP as well? I understand that it's default, but it would be much easier to read

Agreed, that makes sense.

Also both factories unused_udp_port_factory and unused_tcp_port_factory are essencially the same. The only difference is socket_type.

True. My intention was to stay fully backward-compatible. So changing unused_tcp_port_factory to accept a socket type would invalidate the tcp' in its name. But I could extract the implementation into a common function (like with the _unused port' function) and make the fixtures a slim wrapper.

Additionally, please, spread documentation to fixtures.

Also agreed.

I'll look into the code changes soon.