pid1: fix cleanup of stale implicit deps based on /proc/self/mountinfo

The problem was introduced in a3742204:
we have a unit which has a fragment, and when we'd update it based on
/proc/self/mountinfo, we'd say that e.g. What=/dev/loop8 has origin-fragment.
This commit changes two things:
- origin-fragment is changed to origin-mountinfo-implicit
- when we stop a unit, mountinfo information is flushed and all deps based
  on it are dropped.

The second step is important, because when we restart the unit, we want to
notice that we have "fresh" mountinfo information. We could keep the old info
around and solve this in a different way, but keeping stale information seems
inelegant.

Fixes #11342.

(cherry picked from commit c52c2dc6)