Replace more FFmpeg references by Libav.

doc/muxers.texi

libavcodec/dsputil.c

libavcodec/dv.c

libavcodec/dxva2.h

libavcodec/dxva2_h264.c

libavcodec/jpeglsdec.c

libavcodec/libdirac.h

libavcodec/libdiracdec.c

libavcodec/libschroedingerdec.c

libavcodec/libtheoraenc.c

libavcodec/mimic.c

libavcodec/truemotion2.c

libavcodec/utils.c

libavcodec/vaapi_h264.c

libavcodec/vaapi_mpeg4.c

libavcodec/vaapi_vc1.c

libavcodec/version.h

libavcodec/vorbis_enc.c

libavcodec/wmavoice.c

libavfilter/vf_fade.c