movenc: add faststart option for web streaming

Faststart moves the moov atom to the beginning of the file and rewrites
the rest of the file after muxing is complete.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit 'f8ef91ff3d6bb83d601d816ef9368f911021c64b'

* commit 'f8ef91ff3d6bb83d601d816ef9368f911021c64b':
  movenc: add faststart option for web streaming

Conflicts:
	doc/muxers.texi
	libavformat/movenc.c
	libavformat/movenc.h

Merged-by: Michael Niedermayer <michaelni@gmx.at>