stream_file.c contains some code meant for forward seeking with pipes.
This simply reads data until the seek position is reached. Move this
code to stream.c. This stops stream_file from doing strange things
(messing with stream internals), and removes the code duplication too.
We also make stream_seek_long() use the new skip code. This is shorter
and much easier to follow than the old code, which basically did strange
things.