Convert "get own URLs" code into nicer chunks, and test those.
This minimises the code that actually depends on the available network
interfaces, and increases test coverage. Found a few bugs too.
Due to the way SSDP works, Flamenco Manager needs to know its own URL,
where the Workers can reach it. These URLs are now found, and since there
can be multiple (like IPv6 + IPv4) they are all sent in a SSDP
notification as ;-separated strings.
