Specify unsigned-byte element-type when opening mp3 file.

This was causing read-byte to immediately fail in sbcl when attempting
to read the mp3 file in play-current.
Set buffer size to metadata-interval in play-current-buffered-io.

The original code attempted to use an undefined symbol ``size'' as the
buffer size. It might be better to add a binding for ``size'' based on
the underlying filesystem's block size, but the default value for
metadata-interval of 4096 is correct for most filesystems.