mixer(3): Implement mixer_get_path() function

This is better than hardcoding device paths in mixer applications.

Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45275

(cherry picked from commit 67c89b21b95601c01bafe5a0c518d320a39111c0)