config/types: prevent files/links/dirs from conflicting with systemd units or dropins

[marmijo]

Fixes Ignition#881.

Users can specify files/links/dirs within /etc/systemd/system that would conflict with systemd units or dropin files at the same location. Add validation to prevent this configuration conflict.

See machine-config-operator#1203 for the decision to add this validation to Ignition.

[marmijo]

This validation may need to be updated to include the fix for #1457 whenever that is implemented.

[marmijo]

I updated this PR to add validation to versions of Ignition in config

[marmijo]

These errors in config_test.go are what I saw locally. I'll add back in the other lines explicitly setting these other members to be empty. https://github.com/coreos/ignition/actions/runs/3191845930/jobs/5208635971#step:5:30

[bgilbert]

Some small fixes, but this generally looks good. 👍

Please don't include substantive changes and a rebase in the same force-push; it makes the interdiff hard to read. We generally try to rebase, force-push, and then make substantive changes (or the other way around).

[marmijo]

Please don't include substantive changes and a rebase in the same force-push; it makes the interdiff hard to read. We generally try to rebase, force-push, and then make substantive changes (or the other way around).

I didnt know think about how it would affect the PR in that way. Thanks for the feedback!