Completely flush the upstream buffer at startup, before attempting to
fetch a new task. These updates could impact any task on the Manager side,
and first flushing the buffer before appending new updates also seems
like a good idea.
Move the UPnP/SSDP Manager autodiscovery code into from `main.go` into the
`worker` package. This also means changing the error handling a bit, as
only the `main.go` file is allowed to do `log.Fatal()`.
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.
