avcodec: add codec_whitelist

This allows restricting decoders to a list of needed ones for improved security

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

This allows restricting demuxers to a list of needed ones for improved security
Note, some demuxers themselfs open other demuxers, these are only restricted if
AVOptions are forwarded to them. Please check that your code does that.

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