Service interaction fixes for debian

Hi, quite a while ago we discussed about bug 895821 and a while after I realized one issue that was left was happening on removal of chrony. I fixed that up in Ubuntu and forgot about it at first.

But today I looked at a merge of the most recent version and I retested the case. Doing so I realized the change still is needed and in addition found another issue that we should fix as well.

So I decided to polish all that to be used for this PR. I think the commits have enough text to explain the reasoning even further. I'd be happy if that could help Debian as well, let me know what you think about them.

P.S. intentionally a PR for debian/experimental as this clearly is not buster but bullseye material.