input: don't override "time" and "position" when user seeks
Problem discussed here: https://mailman.videolan.org/pipermail/vlc-devel/2016-June/108217.html
We'll try the solution 3/ first: "The variables are removed completely. Different means must be used both to request seek and to retrieve the current time."
Question about 3:
Should input_get{Position|Time} always return the real time ? If so, we need to a add a callback to be notified when the seek is done (called after first Demux() that follow a seek). This will be needed by UIs (in order to display the seek time instead of the real time until we receive that seek_done callback).