beginnings of virtual playback channel support

instead of using two malloced arrays for storing channel lists, use an
slist.  convert the sndstat device to use sbufs and optionally provide more
detail about channel state.

vchans are software mixed playback channels.  they are not enabled by this
commit.  they use the feeder infrastructure to emulate normal playback
channels in a manner transparent to applications, whilst providing as many
channels are desired, especially suitable for devices with only one hardware
playback channel.  in the future they will provide additional features.

those wishing to test this functionality will need to add vchan.c to
sys/conf/files and use 'sysctl -w hw.snd.pcm0.vchans' to enable it.

blocksize and auto-rate selection are not yet supported.

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