vorbis_parser: use a dedicated AVClass for logging

Currently, the API takes an external AVCodecContext, which is used only
for extradata and logging. This change will allow to it to work without
an AVCodecContext in the following commits.
vorbis_parser: add an AV prefix to VorbisParseContext

This is done in preparation for making it public.
lavc: add a public API for parsing vorbis packets.

It is required by (at least) the ogg demuxer.

Mark the current semi-public apriv API for removal.
lavc,lavf: switch to the new vorbis parse API
Merge commit '8747926'

* commit '8747926':
  vorbis_parser: use a dedicated AVClass for logging

Conflicts:
	libavcodec/vorbis_parser.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '6896f95b2483e52e717e2c75a4fd24fcb0e14b67'

* commit '6896f95b2483e52e717e2c75a4fd24fcb0e14b67':
  vorbis_parser: add an AV prefix to VorbisParseContext

Conflicts:
	libavcodec/vorbis_parser.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '5e80fb7ff226f136dbcf3fed00a2966bf8e9bd70'

* commit '5e80fb7ff226f136dbcf3fed00a2966bf8e9bd70':
  lavc: add a public API for parsing vorbis packets.

Conflicts:
	doc/APIchanges
	libavcodec/Makefile
	libavcodec/version.h
	libavcodec/vorbis_parser.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/export av_vorbis_parse_frame_flags()

The other functions where changed in the previous merge

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '2f3fadfbe3c6ad52fad5c614b6067c5401227959'

* commit '2f3fadfbe3c6ad52fad5c614b6067c5401227959':
  lavc,lavf: switch to the new vorbis parse API

Conflicts:
	libavformat/oggparsevorbis.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/oggparsevorbis: return proper error code from vorbis_header()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/oggparsevorbis: Check that initialization succeeded before declaring the end of headers

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