core/varlink: make sure we setup non-serialized varlink sockets

core/varlink: make sure we setup non-serialized varlink sockets

Before this PR, if m->varlink_server is not yet set up during
deserialization, we call manager_setup_varlink_server rather than
manager_varlink_init, the former of which doesn't setup varlink
addresses, but only binds to methods. This results in that
newly-added varlink addresses not getting created if deserialization
takes place.

Therefore, let's switch to manager_varlink_init, and add some
sanity checks to it in order to prevent listening on the same
address twice.

Fixes #29373

Replaces #29421