Resolve "Fix post-upgrade init scripts cleanup"

This MR has 2 commits for easier review (but I can totally split it up in 2 MRs on reviewer request):

• 9851c0cb:

  This commit fixes this issue, by doing two things:

  • move cleanup code from 8166dc04 to S98cleanup
  • run S98cleanup on shutdown, so that the cleanup will be performed during last shutdown of version OLD (rather than during first startup of version NEW)

• 71c42ded:

  This commit makes a few style and POSIX-compliance tweaks to S98cleanup to be able to execute it with ash. It also adds an entry in the init log when a file deletion actually happens.

Closes #957 (closed)

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