Misc. cleanup.
Theora is only based on VP3.
Don't have aliases with the same name (short or long) as actual encoders.
Use better name for Libav encoders.

This is more consistent with the other encoders:

H.264  (x264)    -> libx264
Theora (theora)  -> libtheora
AAC    (faac)    -> libfaac
Vorbis (vorbis)  -> libvorbis
AC3    (avcodec) -> libavcodec
etc.
Cosmetics: re-indent.
While we have a legacy encoder by the name "AC3", "ac3", it's also a generic name so move it.