Dragonfly always passes a flag for every IO operation depending whether

Dragonfly always passes a flag for every IO operation depending whether
the mode of the operation or of the fd is set to NBIO, but it doesn't
pass down fcntl() changes to the drivers. So, if you open /dev/dsp with
NONBLOCK and later fcntl it to blocking, the sound driver won't be aware
of this fact.

Fix: don't maintain this setting in the sound driver.

Requested and tested by corecode@.