mixer(8): Allow full PCM device names as input for the -d option

The -d option is a wrapper around hw.snd.default_unit. Currently
mixer(8) expects the option argument to be just the unit's number (e.g
pcm0 -> 0). To avoid confusion, allow full device names of the form
"pcmN" as well.

While here, improve the -d option's description in the man page.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, imp
Differential Revision: https://reviews.freebsd.org/D43794

(cherry picked from commit 75be886e58dc237b633104fc9cf8d7d1285e4003)