Introduce get_header() for ogg/vorbis.

Introduce get_header() for ogg/vorbis.

Define the get_header() method for the ogg vorbis audio format handler
which replaces ogg packet #2 by a dummy packet which contains no meta
data.

We call ogg_get_file_info() with afhi == NULL which only passes the
first three ogg packets to the given callback but does not compute
the chunk table. Ogg packet #1 and #3 are copied unmodified into the
header.