Use systemctl for shutdown/reboot functions.

[Hooverdan96]

As suggested by @phillxnet here: #2567 creating a separate issue for this.

After the move of some paths from the settings.py to constants.py, I noticed the shutdown path.

In the spirit of simplification, I am proposing to use systemctl poweroff/systemctl reboot instead of the shutdown command/path to get rid of another setting/path to keep track of.

[phillxnet]

@Hooverdan96 Thanks.

Lets address this once we have our current Py3.6 Milestone sorted.

[phillxnet]

Linking to the related proposal by @FroggyFlox to replace our direct interaction with systemd all-together:
#2680

I.e. we might use this issue to test-run/trial our adoption of pystemd, whilst simultaneously removing said low-level path entries: and using systemd shutdown/reboot commands (but via psystemd).

[phillxnet]

@Hooverdan96
Re:

Lets address this once we have our current Py3.6 Milestone sorted.

OK, so we've just closed our Py3.11 Milestone - I think that qualifies now :) .

This one should be fairly straigh-forward - and I don't think it should necessarily await #2680 as that is a far larger scope and this is a step within that scope: as I see it.