core: add unit_dbus_interface_from_type() to unit-name.h

core: add unit_dbus_interface_from_type() to unit-name.h

Let's add a way to get the type-specific D-Bus interface of a unit from
either its type or name to src/basic/unit-name.[ch]. That way we can
share it with the client side, where it is useful in tools like cgls or
machinectl.

Also ports over machinectl to make use of this.