Instead of sending the current process an interrupt signal, use a dedicated
channel to signal the wish to shut down. The main function responds to that
channel closing by performing the shutdown.
This solves an issue where the Worker would not cleanly shut down on
Windows when `offline` state was requested by the Manager.
Within the shutdown procedure, signing off is now the last thing the
worker does. This makes things more consistent from the Manager's point
of view (like receiving last-second log entries while the Worker is still
online).
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.
