Add OSS mixer interface (from Nicholas FitzRoy-Dale <wzdd@lardcave.net>):

* add oss_mixer_t and OSSMixerType
  * add newossmixerobject(), oss_mixer_dealloc(), ossopenmixer()
  * add _do_ioctl_1_internal() to support mixer ioctls
  * add mixer methods: oss_mixer_{close,fileno,channels,stereo_channels,
    rec_channels,getvol,setvol,getrecsrc,setrecsrc}()
  * add oss_mixer_methods list
  * add oss_mixer_getattr() (why?!)
  * export SOUND_MIXER_* constants from soundcard.h