stream_file: uncrustify
stream_avdevice: remove redundant dummy callback
stream: move file forward skipping to common stream implementation

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.
stream: don't require streams to set s->pos in seek callback

Instead, set s->pos depending on the success of the seek callback.
stream: allow potentially faster skipping

Instead of always skipping in STREAM_BUFFER_SIZE blocks, allow an
arbitrary size. This allows - in theory - faster forward seeking in
pipes.

(Maybe not a very significant change, but it reduces the number of
things that depend on STREAM_BUFFER_SIZE for no good reason. Though
we still use that value as minimum read size.)
demux_raw: read multiple frames per packet

The rawaudio demuxer read one frame per packet, basically a few bytes,
which caused insane overhead. (I found this when I couldn't play raw
audio without dropouts when using -v, which printed a line per packet
read.)

Fix this and read 1 second of audio per packet. This is a regression
since cfa5712 (merging of demux_rawaudio and demux_rawvideo).
demux: check whether stream is selected in demux_get_next_pts()

Otherwise, this would just try to demux a good chunk of the file, even
though the operation can't succeed anyway.

This caused some pretty strange issues, where perfectly valid use cases
would print a "Too many packets in the demuxer packet queue..." message.
demux: remove unused audio_delay parameter from demux_seek()

Used to be needed by demux_avi.