mixer(8): Use new mixer if we change the default unit

If we use the -d option to change the default unit, close the current
mixer and open the one we set as the default to avoid printing and
applying changes (if any) to the old one.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D43809