core/unit: search shared namespace in transitive relation of JoinsNamespaceOf=

core/unit: search shared namespace in transitive relation of JoinsNamespaceOf=

Previously, dependency chain of JoinsNamespaceOf= did not work, e.g.
- a.service has JoinsNamespaceOf=b.service
- b.service has JoinsNamespaceOf=c.service
if, first c.service, next a.service, finally b.service is started,
then a.service is not joined to the namespace of c.service. And, as
mentioned in the document, the namespace used by b.service is not
deterministic.

This makes when searching exsiting namespace to be joined, all units in
the transitive dependency of JoinsNamespaceOf= are checked.