manager: Freeze/Thaw: Don't fail units w/o cgroup

manager: Freeze/Thaw: Don't fail units w/o cgroup

Previously, it was impossible to freeze or thaw a slice if it is an
ancestor to a unit that had no running cgroup (i.e. a service with
RemainAfterExit=yes). Instead of failing with EBUSY (which would
confusingly get reported as "Unit has pending job") we just silently
no-op.

I noticed this because we now have user-runtime-dir@.service, which
would make it impossible to freeze/thaw user.slice or user-<UID>.slice