Install systemd unit to run cleanup on shutdown or reboot

[agracey]

Is your feature request related to a problem? Please describe.
When rebooting a node, the system shows

[FAILED] Failed unmounting /etc
[FAILED] Failed unmounting /var

for a few minutes before timing out and rebooting.

This makes upgrades take longer than they should and can cause longer than desired service interruptions when hardware is attached to specific nodes (e.g. an IoT gateway for manufacturing)

Describe the solution you'd like
I would like for the REK2 installer to include a systemd unit file that runs rke2-killall.sh when a shutdown or reboot is requested. This has been discussed as a work around previously in: k3s-io/k3s#7362 (comment)

Describe alternatives you've considered

• Just waiting means an unpredictable outage
• Running rke2-killall.sh manually means that any tooling that manages the system needs to know about it (which is error prone)
• Writing the service yourself means that some users are going to not see the documentation and raise new issues

Additional context
This is a companion issue to k3s-io/k3s#10431