a2_Close() now does things in the right order

There were occasional segfaults in the 'rtsubstate' test, and
Valgrind wasn't happy at all with how a2_Close() cleaned up.
It turns out a2_Close() just unloaded everything first thing,
when closing a master state!

Now, it disables the audio callbacks of the master state and
any substates, *before* unloading everything. Everyone likes
this a lot better, for some reason.

Closes #111.