Header size can reach UINT32_MAX.

Unlike stream_Size(), this helper distinguishes between unknown and
zero sizes.

Due to backward command line compatibility, zero size is not possible
with the old imem. Consider using the proper LibVLC API.

So far, it was returning zero on error. That prevented distinction
between an empty file and a file with unknown size.

Regression from previous time fix. Test url
http://wams.edgesuite.net/media/SintelTrailer_Smooth_SeekDemo/sintel_trailer-1080p.ism/manifest%28format=mpd-time-csf%29

This removes the singleton, fixes the xib, and initializes
the dialog provider class at the correct time. dialog_Register
will be called only once now.

Since we're dealing with 3 different units (time, mtime and scaled time)
things have gone wrong when merging hls and dash code.

Should fix the crazy manifest/playlist updates

introduced by cf1d5561

avformat needs nul padding after the probe data.