it instead of the OS-specific <linux/soundcard.h> or <machine/soundcard.h>.

Mixers devices have an ioctl-only interface, no read/write -- so the
flags passed to open() don't really matter.  Thus, drop the 'mode'
parameter to openmixer() (ie. second arg to newossmixerobject()) and
always open mixers with O_RDWR.