core: support "nsdelegate" cgroup v2 mount option (#6294)

core: support "nsdelegate" cgroup v2 mount option (#6294)

cgroup namespace wasn't useful for delegation because it allowed resource
control interface files (e.g. memory.high) to be written from inside the
namespace - this allowed the namespace parent's resource distribution to be
disturbed by its namespace-scoped children.

A new mount option, "nsdelegate", was added to cgroup v2 to address this issue.
The flag is meangingful only when mounting cgroup v2 in the init namespace and
makes a cgroup namespace a delegation boundary.  The kernel feature is pending
for v4.13.

This should have been the default behavior on cgroup namespaces and this commit
makes systemd try "nsdelegate" first when trying to mount cgroup v2 and fall
back if the option is not supported.

Note that this has danger of breaking usages which depend on modifying the
parent's resource settings from the namespace root, which isn't a valid thing
to do, but such usages may still exist.