bridge: Don't complain about adding/removing rules if cancelled

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Don't cancel DBus rule removals when disposing channel

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Don't put path_namespace in match rules unnecessarily

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
base: Provide defaults for dbus client.proxy() and client.proxies() args

This helps avoid stuttering in cases like systemd where the interface,
bus name, and path are all pretty much identical

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Make the dbus "system" bus the default

Most people will be connecting to the system bus, so make this the
default for the dbus-json3 open control message.

Closes #1482
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>