core: pass Unit into cgroup_context_apply() and use log_unit*()

core: pass Unit into cgroup_context_apply() and use log_unit*()

cgroup_context_apply() and friends take CGroupContext and cgroup path as input
and has no way of getting back to the associated Unit and thus uses raw cgroup
path for logging.  This makes the log messages difficult to track down.
There's no reason to avoid passing in Unit into these functions.  Pass in Unit
and use log_unit*() instead.

While at it, make cgroup_context_apply(), which has no outside users, static.
Also, drop cgroup path from log messages where the path itself isn't too
interesting and can be easily obtained from the unit.