lavfi/curves: add presets support.

Except for the vintage preset, the values are defined by Lou Logan based
on the ones found in Adobe Photoshop CS6.

Signed-off-by: Clément Bœsch <ubitux@gmail.com>
Signed-off-by: Lou Logan <lou@lrcd.com>
lavfi/curves: support preset shorthand.
lavfi/curves: add forgotten strong_contrast preset.
tools: add audio normalize script example.
ffmpeg_opt: avoid strcpy()

This fixes a warning on openbsd and is more robust

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