DemuxConn.__init__: you can't assume the *last* 6 bytes are BUPMUX.

DemuxConn.__init__: you can't assume the *last* 6 bytes are BUPMUX.

The actual muxed data might arrive immediately after it, and since we're not
buffering that, we have to read one byte at a time.

(Buffering would be more efficient if we expected this to happen frequently,
but it shouldn't.)

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>