Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Move code from the Vorbis and FLAC decoder plugins.

Eliminates the functions _new() and _free().

Support these PCM formats natively, instead of letting libsndfile
convert everything to 32 bit.

Prepare for other sample formats.

That attribute was uninitialized before, which could crash
libavformat.

See Debian bug 760669

Log call was added to the wrong branch.

Fixes regression by commit ca1a1149

Simply return an empty WritableBuffer, not a nulled one.

The unit of gme_info_t::length is milliseconds, not centiseconds.