[Call for testing] sound: Implement /dev/dsp as a router device

I've been working on a patch [1] to re-implement /dev/dsp as a
router/virtual device. The commit message in the review explains all the
high-level details and reasons behind the patch. This is the first of a
series of patches I'm working on.

For regular users, the main functional difference is that with this
patch, /dev/dsp is an actual router device (like when using
virtual_oss), which means that we have hot-swapping (i.e., changing the
default device and routing audio to it during IO without needing to
close and re-open /dev/dsp) built into sound(4).

Feel free to test the patch and please let me know if there's anything
that breaks or could be improved. You can play around with it just by
recording/playing audio as you always do, try some hot-swaps,
hot-(un)plugs, and anything that can stress test it. Just make sure you
open /dev/dsp, which should be the default for most applications anyway.

Christos

[1] https://reviews.freebsd.org/D49216