* channels() -> devices()
  * stereochannels() -> stereodevices()
  * recchannels() -> recdevices()
  * getvol() -> get()
  * setvol() -> set()
This is for (slightly) more consistency with the OSS ioctl names
(READ_DEVMASK, READ_RECMASK, READ_STEREODEVS).

Also make sure the C function names correspond more closely to the
Python method names for mixer methods.