Support for DragonFlyBSD and Asus Xonar Essence ST card

Works for sure for DragonFlyBSD with ST card. Can be built on FreeBSD, but functionality is not tested (I don't have the card on FreeBSD box).

Minor changes:
* DAC is muted while switching audio output. It helps to avoid a loud "click" sound in headphones (which I believe is called a pop in source)