httpd: Start new clients only at keyframes.

When the mux is capable of marking blocks as containing keyframes
(currently only the avformat mux), start new clients only on such
frames. This makes sure they do not get anything they might not
decode at the beginning of the stream.

This patch completes WebM streaming support.

Signed-off-by: Rafaël Carré <funman@videolan.org>
NEWS: mention x262
Contribs: Use x262 from VideoLAN
Mention WebM streaming in NEWS