Reject multiple headers of the same type.

Reject multiple headers of the same type.

A common application pattern is to call vorbis_synthesis_headerin()
 and count how many times it succeeds.
If you feed it multiple valid comment headers, they will all
 succeed, meaning you can be fooled into think you have a valid
 Vorbis file despite never seeing a setup header.
This patch makes libvorbis reject multiple headers of the same type,
 preventing this from occurring.

svn path=/trunk/vorbis/; revision=19426