sout: allow duplicate outputs to be merged

sout: allow duplicate outputs to be merged

The stream_out_t chain creation is modified: all modules are created by
the core (or by stream_out_duplicate) instead of being created by the
previous module.

sout_StreamChain{New,Delete} replace sout_Stream{New,Delete} to handle
modules chains instead of individual modules

sout_Stream{New,Delete} are still used by those new functions but made
static inside stream_output.c

Remove now unneeded psz_chain from struct sout_instance_t
Replace pointer to chain of next module by pointer to next module in
struct sout_stream_t

Example use:

vlc --sout-all input.mp4 --sout
"#duplicate{dst=transcode{vcodec=mp2v},select=es=0,dst=transcode,select=es=1}:std{...}"

(dst=transcode without acodec/vcodec is a hack to pass the encoded stream to
stream_out_standard without transcoding)