cgroup: also indicate cgroup delegation state in user-accessible xattr

cgroup: also indicate cgroup delegation state in user-accessible xattr

So far we set the "trusted.delegate" xattr on cgroups where delegation
is on. This duplicates this behaviour with the "user.delegate" xattr.
This has two benefits:

1. unprivileged clients can *read* the xattr. "systemd-cgls" can thus
   show delegated cgroups as such properly, even when invoked without
   privs

2. unprivileged systemd instances can set the xattr, i.e. when systemd
   --user delegates a cgroup to further payloads.

This weakens security a tiny bit, given that code that got a cgroup
delegated can manipulate the xattr, but I think that's OK, given they
have a higher trust level regarding cgroups anyway, if they got a
subtree delegated, and access controls on the cgroup itself are still
enforced. Moreover PID 1 as the cgroup manager only sets these xattrs,
never reads them — the xattr is primarily a way to tell payloads about
the delegation, and it's strictly this one way.