The dev.pcm.N.mode sysctl lets us know the playback/recording mode of a sound device (none, play, rec, play+rec).
It's more consistent _both_ with the rest of the flag values in the file
(e.g SD_F_*) and with how flags are handled in userland; now we
can AND the value of mode with any of the flags. If PCM_MODE_NONE's
value is 0x00, we have to specifically check if mode == 0 too see if
PCM_MODE_NONE is set, which makes things a bit more messy --
at least in my opinion.