sound: Shorten channel names

The current channel naming convention is:
pcmX:[virtual_]play|record:dspX.[v]p|rY

• pcmX and dspX share the same unit numbers
• "[v]p|r" gives us the same information as "[virtual_]play|record"

Remove the redundant information, so that the channel names become more
readable: dspX.[virtual_]play|record.Y

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46836