packages: Add a sound package

Put virtual_oss, /etc/devd/snd.conf and the other audio-related tools
into a new "sound" package. Don't create a separate -lib package,
since it's unlikely someone will want mixer(3) without mixer(8).

Put the sound package in the optional set rather than minimal, since
it's not actually required for audio hardware support, and many systems
(including nearly all servers) won't want it installed.

MFC after: 3 seconds
Reviewed by: christos
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52823

(cherry picked from commit 1b806e607f52285f973fb1106491668fd28da6a4)