Make it possible to siphon out a single MJPEG stream.

The URL for this is /feeds/N.mp4 (although the .mp4 suffix will be ignored
in practice), where N is the card index (starting from zero). You are allowed to
use a feed that is not part of the “regular” multicam MJPEG input.

This can be used for remote debugging, recording of a given card with wget,
streaming a specific card as an aux, and probably over things.