Fix systemd service

[jfhbrook]

The systemd service is currently broken. This branch fixes the systemd unit, and makes progress towards full functionality.

The good news is that the service starts:

Feb 06 10:48:17 stardeck systemd[1]: Started plusdeck.service - Plus Deck 2C PC Cassette Deck...
░░ Subject: A start job for unit plusdeck.service has finished successfully
░░ Defined-By: systemd
░░ Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
░░ 
░░ A start job for unit plusdeck.service has finished successfully.
░░ 
░░ The job identifier is 199774.
Feb 06 10:48:17 stardeck plusdeckd[1238864]: INFO:plusdeck.dbus.service:Listening on org.jfhbrook.plusdeck /

I can also manage the plusdeck with plusdeckctl using sudo.

However, unprivileged access is eluding me. The intent is that unprivileged users in the plusdeck group may have full access to the service. Currently what works is listening to signals (plusdeckctl subscribe), as well as property access (plusdeckctl config show). Unfortunately, method calling (plusdeckctl eject) is not working.

I believe the issue lies with systemd, either with some capacities I have to manually set, or with some settings I need in polkit. For more information, see these discussion threads:

• https://unix.stackexchange.com/questions/790750/dbus-policy-that-allows-group-to-access-system-service
• https://discussion.fedoraproject.org/t/dbus-policy-that-allows-group-to-access-system-service/144265

My intent is to merge and release this as-is, and delegate fixing unprivileged access to a secondary ticket and PR.

[jfhbrook]

I figured out that the default bus is the system bus when started with a non-user systemd unit: https://www.freedesktop.org/software/systemd/man/latest/sd_bus_default.html based on me being able to get past a "where is the bus tho" error, I seem to understand this correctly.