lavfi: add compand audio filter

This patch adds the `compand` audio filter from ffmpeg master branch
(currently at 7f0f47b3df) adapted to work with libav.

The filter makes use of `av_strtok` to parse the input parameters, and
it appears that libav has removed that function, so I inlined it into
the filter. Same goes for `av_clipd_c`. Same for
`av_samples_alloc_array_and_samples` except it looks like ffmpeg added
that function recently.