upipe-avfsink: move to upipe_set_option
upipe_probe_uref: add drop feature
upipe-x264: copy extradata if repeat_headers==0
upipe_avcodec_encode: copy extradata
upipe_avfsink: various fixes

- fix extradata muxing
- fix avformat options
Merge pull request #4 from funman/master

upipe-av depends on libavutil
upipe_avfsink: add _get_duration()
clear start flag on entering the ts mux
Merge branch 'master' of http://git.notk.org/bencoh/upipe
ts mux: better resilience when input interrupted
avcdec, x264: make sure dates are monotonic