Keep a copy of the playing queue, status etc. to render the UI without doing imsg in the render path.

This is to help amused-web. Change the ctl to send an extra STATUS after a MODE change to preserve the behaviour.

In the future we could apply some caching to these.

add bufio_free() to free the bufio without re-initialization, add support for writing HTTP-style chunks and add ability to extract data from the read buffer via the bufio_* layer instead of peeking at the internals.