lavfi/values: a bit more consistent coding style.
lavfi/values: add FLAGS where missing in options.
lavfi/values: fix typo and reword stat description entry.
lavfi/values: re-order headers.

System headers belong on top.
lavfi/values: remove some pointless headers.

Also fix the fopen() dependency.
lavfi/values: remove dependency to libavcodec.
lavfi/values: remove some pointless init.

Context is zero-ed.