mixer(8): Implement hot-swapping

Introduce a -V option, which can be used alongside -d (default unit
change), in order to hot-swap devices (i.e switch to them on the fly
without needing to restart the track), in case virtual_oss(8) exists and
is running.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46253