core: introduce UNIT_ATOM_PROPAGATE_STOP_GRACEFUL for PropagatesStopTo=

core: introduce UNIT_ATOM_PROPAGATE_STOP_GRACEFUL for PropagatesStopTo=

Follow-up for 017a7ba4f406adcf69d6b3ec15b9f2d9ed5ad853

Before this commit, when a unit that is restarting propagates stop
to other units, it can also depend on them, which results in
job type conflict and thus failure to pull in the dependencies.

So, let's introduce a new dependency atom UNIT_ATOM_PROPAGATE_STOP_GRACEFUL,
and use it for PropagatesStopTo=. It will enqueue a restart job if
there's already a start job, which meets the ultimate goal and avoids
job type conflict.

Fixes #26839