Squeezlite and MPD need an extra option - no DSD support

Snakeoil assumes all DACs will either support native DSD or DSD Over PCM (DoP). This is an oversight because not all DACs support DSD. And there's no way to configure these DACs in the Snakeoil WebApp!

Need to add a third option to disable DSD support.

Unfortunately user also reported transcoding is not working. This is a different issue and may need to be looked at as well.

First mentioned here: https://community.snakeoil-os.net/t/dsd-playback-on-squeezelite-endpoint-with-pcm-only-dac-schiit-modi/473