fix: uninstall custom signal handlers before shutdown

[holmanb]

Proposed Commit Message

fix: uninstall custom signal handlers before shutdown

Fixes GH-5849
Fixes LP: 2089185

Additional Context

This might address the following bug reports:

https://bugs.launchpad.net/maas/+bug/2089185
#5849

Test Steps

I haven't been able to reproduce either bug, so testing is needed.

Here is are debs for testing (funny name because Github is unaware of the existence of debian files):

noble: cloud-init_24.3.1-1174-gf7bbd23d-1~bddeb_all.tar.gz
jammy: cloud-init_24.3.1-1016-ge3d0bcd4-1~bddeb_all.tar.gz

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[TheRealFalcon]

@holmanb , I'm assuming we're looking for some confirmation that this approach works before reviewing?

[r00ta]

We tested this patch in MAAS but it does not work. As per chat we decided to release a fix in MAAS directly.

So, if this patch was supposed to be used only for MAAS we can close it. Thanks!