af_hdcd: for easier maintenance alongside libhdcd

Mostly just re-arranges some code to make it easier to update this
filter and libhdcd together. filter_frame() is much simpler as a
result.

* use the HDCD detection data structure and functions from libhdcd,
  moved detection code out of filter_frame()
* moved analyze_mode preparation out of filter_frame() into
  hdcd_analyze_prepare(), from libhdcd
* moved some macro definitions to the top so they are all together

Signed-off-by: Burt P <pburt0@gmail.com>