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

[christos]

Summary

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

[christos]

Remove wrong usage info about being able to use both -d and -a at the same
time.