Use lavf's flv demuxer for rtmp/rtmps/... stream types. Letting
generic format probing handle this could work, but with the current
probing implementation it'd at least depend on not-really-guaranteed
details of the stream layer (probing different formats and then
decoding depends on seeking back in between; rtmp streams don't
support such seeking directly so would need to rely on details of
caching behavior).
Change reading data from FFmpeg from url_read() back to
url_read_complete(). url_read_complete() behavior has changed in
FFmpeg and no longer returns an error for partial reads at EOF so it's
usable now. Using it is probably not significantly better than
url_read(), but at least it handles EAGAIN.
stream_ffmpeg used libavformat's url_read_complete() to read data.
However that function returns failure if it did not manage to read the
_full_ amount of data asked for, while the behavior we want is to
return any positive amount of data there was before end of file. This
caused attempts to read the last bytes in a file to fail. Fix by using
url_read() instead of url_read_complete(); even if some reads before
EOF return less than the full amount that should not be a problem.
