Switch to a new version of the Metacube protocol.

Compared to the old version, this fixes many small deficiencies:

 - The header is smaller; 16 instead of 24 bytes. With TS, Metacube actually
   yields noticeable overhead (13% or so), so reducing it by 1/3 is good.
 - The sync no longer overlaps with itself; both starting and ending with
   the same sounds bad if we should ever drop bytes in the middle.
   In fact, it no longer has any repeated characters.
 - The header is checksummed, to avoid cases where a corrupted header
   could cause us to pull in gigabytes of data. (The data is not.)

vlc-metacube.diff has been updated, although it includes another patch
(part of the WebM patch set) since that's what my VLC works from at the moment.