bus: Clarify names of methods that query owned names

It wasn't immediately clear from the names of these method whether they
should return TRUE or FALSE for queued owners other than the primary
owner. Renaming them makes it obvious that the answer should be TRUE.

While I'm there, make the corresponding _dbus_verbose() messages more
precise.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: #270