sound: Move vchan-related code to pcm/vchan.*

pcm/sound.* contains code that should be part of pcm/vchan.*.

Changes:

• pcm_setvchans() -> vchan_setnew()
• pcm_setmaxautovchans() -> vchan_setmaxauto()
• hw.snd.maxautovchans moved to pcm/vchan.c
• snd_maxautovchans declaration moved to pcm/vchan.h and definition to pcm/vchan.c

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45015