sound: Add an example of basic sound application

This is an example demonstrating the usage of the OSS-compatible APIs
provided by the sound(4) subsystem. It reads frames from a dsp node and
writes them to the same dsp node.

Reviewed by: hselasky, bcr
Differential revision: https://reviews.freebsd.org/D30149

(cherry picked from commit 21d854658801f6ddb91de3a3c3384e90f5d920f2)