lavfi: add compand audio filter

This patch adds the `compand` audio filter from ffmpeg master branch
(currently at 7f0f47b3df9d92eea0d1de94df29971b49f10777) adapted to
work with libav.

My downstream library is successfully using this filter to allow the
user to turn up the volume gain beyond 100%:
https://github.com/andrewrk/libgroove/commit/b67b17e091d17bc9eec70f89e61fde90bbee0830

Some things to consider about this filter:

In ffmpeg, an AVFilterLink has the concept of FF_LINK_FLAG_REQUEST_LOOP.
"A filter must set this flag on an output link if it may return 0 in
request_frame() without filtering a frame".

Libav does not have this concept, so I removed the flag. The change
appears to have no effect; however perhaps someone knows better than I do.

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.