opus_afh: Use custom header API to strip comment packet.

Currently we define the header of an ogg/opus file as the first two ogg
pages. This is problematic for two reasons: (a) the metadata packet
(ogg packet #2) may be arbitrary large and should not be sent over
the network as it is not needed for decoding the audio stream, and (b)
the second ogg page may contain, in addition to the metadata packet,
opus encoded audio packets which should not be part of the header.

This patch employs the recently added custom header API to avoid both
problems in the same way the ogg/vorbis audio format handler does:
we create a custom header which contains the unmodified ID header
packet and a dummy metadata packet.