Add content-length header to file streaming as the file length is known
in advance.
Add http Range support to file streaming.

This allows e.g. vplayer to skip/wind within the file.
Add http Range support to file transcoding.

CAVEAT: I have not found a player yet that handles this correctly.
That's because the transcoding doesn't generate timestamps relative
to the position of the file, which players expect.

So it's pretty useless for the moment.