sound: Sort channels by unit number as well

Refactor CHN_INSERT_SORT() to also take into account the unit number of
each channel, while still prioritizing the type, to make the channel
list easier to work with.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46549