lavfi: add compand audio filter

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  lavfi: add compand audio filter

Conflicts:
	Changelog
	doc/filters.texi
	libavfilter/Makefile
	libavfilter/af_compand.c
	libavfilter/allfilters.c
	libavfilter/version.h

The filter is added as new one so as to ease clean merging of its changes
in debug-able steps
See: 6b68e2a43b3407522080be50a2a19cff2f9715ef
Merged-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand: whitespace changes from af_compand_fork

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_compand_fork: avoid strtok_r() so as not to break build

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>