Honor introspection calls on sub path of every exported object

[rrerolle]

This allows introspection based tools such as D-feet to correctly find their way towards exported objects, starting from /

e.g., if two objects are exported under path /a/b/c and /d/e/f, an Introspect call on / will return nodes a and d, while an call on /a will return b, and /d will return e. This allows going from / all the way to /a/b/c and /d/e/f, withouth knowing anything about the exported objects in the first place.

[philips]

@rrerolle Would you mind sending this PR to github.com/godbus/dbus?