Pull up following revision(s) (requested by nat in ticket #825):

share/man/man7/Makefile: revision 1.32
share/man/man9/audio.9: revision 1.46
sys/dev/audio.c: revision 1.453
sys/dev/audio.c: revision 1.454
sys/dev/audio.c: revision 1.455
share/man/man7/audio.7: revision 1.1
share/man/man7/audio.7: revision 1.2
share/man/man7/intro.7: revision 1.24
share/man/man4/audio.4: revision 1.85
distrib/sets/lists/man/mi: revision 1.1586

Fix numbering of vchan mixer controls to correspond to the vchan number in
use.

This makes the numbering of vchans consistient for audioctl and mixerctl.
Expose the audio_info structure of vchan zero(0) the mix ring to allow
setting the hardware gain and balance via audioctl(1) using the -p 0
switch.

It is not possible to influence the hardware gain/blance from the
audio_info structure of vchans 1 onwards.  It is now possible to return
the audio mixers audio format from the audio_info structure of vchan 0 to
ease applications configuring for mmapped play back.

This is conformant to the audio specification posted on tech-kern see:

        "NetBSD Audio Specification 2018"

or audio.7 manual page to be added in a follow up commit.
Add the audio mixer specification to section 7 of the manual.
See posting on tech-kern - "NetBSD Audio Specification 2018."
Update the introduction page to section 7 of the manual noting the audio
spec.

Various improvements, more markup, typo fixes.

Fix over cautious compiler error - setmode may not be initialized.

(martin)