avformat: add option to parse/store ID3 PRIV tags in metadata.

avformat: add option to parse/store ID3 PRIV tags in metadata.

Enables getting access to ID3 PRIV tags from the command-line or metadata API
when demuxing. The PRIV owner is stored as the metadata key prepended with
"id3v2_priv.", and the data is stored as the metadata value. As PRIV tags may
contain arbitrary data, non-printable characters, including NULL bytes, are
escaped as \xXX.

Similarly, any metadata tags that begin with "id3v2_priv." are inserted as ID3
PRIV tags into the output (assuming the format supports ID3). \xXX sequences in
the value are un-escaped to their byte value.

Signed-off-by: wm4 <nfxjfg@googlemail.com>