Implement access_GetParentInput and demux_GetParentInput and use.

Implement access_GetParentInput and demux_GetParentInput and use.

This try to avoid vlc_object_find() as much as possible.
This is conservative, because where there is no associated parent input, we'll try to find in certain cases the parent input. This will probably be
 removed later on. Because yes, there is not necessarily a parent input for access and demux, especially if created from stream_UrlNew().