core: when propagating restart requests due to deps, downgrade restart to try-restart

core: when propagating restart requests due to deps, downgrade restart to try-restart

Previously, if a service A depended on a service B via Requires=, and A
was not running and B restarted this would trigger a start of A as well,
since the restart was propagated as restart independently of the state
of A.

This patch ensures that a restart of B would be propagated as a
try-restart to A, thus not changing its state if it isn't up.

http://lists.freedesktop.org/archives/systemd-devel/2015-May/032061.html