core: fix EBUSY on restart and clean of delegated services

core: fix EBUSY on restart and clean of delegated services

When a service is configured with Delegate=yes and DelegateSubgroup=sub,
the delegated container may write domain controllers (e.g. "pids") into the
service cgroup's cgroup.subtree_control via its cgroupns root. On container
exit the stale controllers remain, and on service restart clone3() with
CLONE_INTO_CGROUP fails with EBUSY because placing a process into a cgroup
that has domain controllers in subtree_control violates the no-internal-
processes rule. The same issue affects systemctl clean, where cg_attach()
fails with EBUSY for the same reason.

Add unit_cgroup_disable_all_controllers() helper in cgroup.c that clears
stale controllers via cg_enable(mask=0) and updates cgroup_enabled_mask to
keep internal tracking in sync. Call it from service_start() and
service_clean() right before spawning, so that resource control is preserved
for any lingering processes from the previous invocation as long as possible.