bus-proxyd: explicitly address messages to unique and well-known name

In order to check for matching policy entries at message transfers, we
have to consider the following:

* check the currently owned names of both the sending and the receiving
peer. If the sending peer is connected via kdbus, the currently owned
names are already attached to the message. If it was originated by the
connection we're proxying for, we store the owned names in our own strv
so we can check against them.

* Walk the list of names to check which name would allow the message to
pass, and explicitly use that name as destination of the message. If the
destination is on kdbus, store both the connection's unique name and the
chosen well-known-name in the message. That way, the kernel will make sure
the supplied name is owned by the supplied unique name, at the time of
sending, and return -EREMCHG otherwise.

* Make the policy checks optional by retrieving the bus owner creds, and
when the uid matches the current user's uid and is non-null, don't check
the bus policy.