Delegated restarter support

[netbsduser]

InitWare needs support for delegated restarters.

[netbsduser]

Originally I wanted to do this by adding a new .delegated unit type and generic code there. I now feel it might be better to allow dynamic extension of the unit types at runtime, maybe with shared libraries that implement the unit vtable. To provide for the trivial case, generic functions should be provided for use, which simply delegate responsibility to a delegated restarter on the D-Bus (or via private bus to manager?)