bridge: Don't put path_namespace in match rules unnecessarily
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
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
WIP debug build failure