Draft: main/postmarketos-base-ui: systemd presets and split into init system specific subpackages

The whole ordeal is split over multiple commits. Not tested yet, so drafting.

This originally bases on @craftyguy 's MR !5182 (closed) . I have made new commits as I experimented around with a few things. Should I set you as the original author or co-authorship for them?

Notes:

• iwd-service is missing systemd-services. doesn't block this tho.
  • added in !5204 (merged)
• does this regress existing installs?
• does this affect new pmbootstrap installations?
• does this work on systemd as intended? (i'll include this mr in the jane400/systemd-test-presets branch for testing)