doc/APIchanges: fill in missing hashes and dates
doc/APIchanges: mark the release 10 branch point
wavenc: use codec descriptors to get the codec name

Also, return a proper error code.
avconv: set the "encoder" tag when transcoding
movenc: use the "encoder" metadata tag to write stsd Compressorname

This mirrors the demuxer behaviour and avoids accessing
AVCodecContext.codec, which should not be done in muxers.