mux: mp4: add fragmented mp4 muxer

mp4stream tries to build stream with 1.5sec runs.
mp4frag aims files by creating a compatible index.

Might need for transcode tests (fragments are 1.5 sec only).
--sout-x264-keyint=30 --sout-avcodec-strict=-2

Streaming done using http, with fragment start used as iframes.