pid1: add D-Bus API for removing delegated subcgroups

pid1: add D-Bus API for removing delegated subcgroups

When running unprivileged containers, we run into a scenario where an
unpriv owned cgroup has a subcgroup delegated to another user (i.e. the
container's own UIDs). When the owner of that cgroup dies without
cleaning it up then the unpriv service manager might encounter a cgroup
it cannot delete anymore.

Let's address that: let's expose a method call on the service manager
(primarly in PID1) that can be used to delete a subcgroup of a unit one
owns. This would then allow the unpriv service manager to ask the priv
service manager to get rid of such a cgroup.

This commit only adds the method call, the next commit then adds the
code that makes use of this.