On some VBR files, the single-step interpolation was very inaccurate
and inacceptable.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/720

When calling OggSeekFindEOS() from inside a OggVisitor callback, then
the #InputStream may be in the middle of an Ogg packet, and the newly
initialized #ogg_sync_state will not be able to load it without the
help of ogg_sync_pageseek().  By passing "synced=false" to
OggSeekFindEOS(), we force the use of ogg_sync_pageseek() even when
not actually seeking.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/719

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.