core: PrivateUsers=true for (unprivileged) user managers

core: PrivateUsers=true for (unprivileged) user managers

Let per-user service managers have user namespaces too.

For unprivileged users, user namespaces are set up much earlier
(before the mount, network, and UTS namespaces vs after) in
order to obtain capbilities in the new user namespace and enable use of
the other listed namespaces. However for privileged users (root), the
set up for the user namspace is still done at the end to avoid any
restrictions with combining namespaces inside a user namespace (see
inline comments).

Closes #10576