core: fix detection whether per-unit drop-ins changed

core: fix detection whether per-unit drop-ins changed

This fixes fall-out from 6d10d308c6cd16528ef58fa4f5822aef936862d3.

Until that commit, do determine whether a daemon reload was required we compare
the mtime of the main unit file we loaded with the mtime of it on disk for
equality, but for drop-ins we only stored the newest mtime of all of them and
then did a "newer-than" comparison. This was brokeni with the above commit,
when all checks where changed to be for equality.

With this change all checks are now done as "newer-than", fixing the drop-in
mtime case. Strictly speaking this will not detect a number of changes that the
code before above commit detected, but given that the mtime is unlikely to go
backwards, and this is just intended to be a helpful hint anyway, this looks OK
in order to keep things simple.

Fixes: #3123