The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
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.
