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

mixer(3): Do not hardcode "/dev/mixer"

We have BASEPATH defined.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D43812

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

mixer(3): remove redundant argument in _mixer_readvol()

There's no reason to pass the mixer as an argument since we can fetch it
from mix_dev. No functional change intended.

Reviewed by: markj, hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38076

mixer(3): Revert: "Fix for header file pollution."

Differential Revision: https://reviews.freebsd.org/D33732
Submitted by: christos@
MFC after: 1 week
Sponsored by: NVIDIA Networking

mixer(3): Fix for header file pollution.

Differential Revision: https://reviews.freebsd.org/D33732
Submitted by: christos@
MFC after: 1 week
Sponsored by: NVIDIA Networking

mixer(3): Fix spelling in comment.

s/MIX_STATUS_XXX/MIX_MODE_XXX/g

Differential Revision: https://reviews.freebsd.org/D32548
Submitted by: christos@
Sponsored by: NVIDIA Networking

mixer(3): Add support for controlling mixer mute and volume on feeder channels.

PR: 258711
Reported by: jbeich@FreeBSD.org
Differential Revision: https://reviews.freebsd.org/D31636
Sponsored by: NVIDIA Networking

Implement and use new mixer(3) library for FreeBSD.

Wiki article: https://wiki.freebsd.org/SummerOfCode2021Projects/SoundMixerImprovements
This project was part of Google Summer of Code 2021.

Submitted by: christos@
Differential Revision: https://reviews.freebsd.org/D31636
Sponsored by: NVIDIA Networking