More seek support.

[Ryan C. Gordon]

I've implemented seek routines for the smpeg, ogg, aiff, wav-adpcm, voc,
and au decoders.

This leaves mpglib.c (which is probably worth leaving unseekable, unless
someone wants to grok mpglib itself to see how to tackle this), shn.c
(ugh), quicktime.c (anyone?) and mikmod.c. Is there a mailing list or
something where we can ask about mikmod seeking?

There were some other minor fixes; check the CHANGELOG.

That 1.0 release is getting really close, now.

Enjoy!

--ryan.