ffmpeg: do not build bottle with --enable-nonfree

faac, fdk-aac, and openssl are all distributed under GPL-incompatible licenses; if ffmpeg is built with these libraries, it cannot legally be redistributed. The current default options seem to derive from before we were distributing any binary packages, and so we didn't have to worry about licensing.

This replaces faac in the default build with libvo-aacenc, which is not quite as good but which is GPL3-licensed.