Move code from the Vorbis and FLAC decoder plugins.

To replace g_strdup().

Replaces GLib's g_ascii_strncasecmp().

Reduce heap allocator overhead.

Merge duplicate data.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.