shutdown ao on library removal

[daurnimator]

There was previously ineffective code (removed in a5a9b7b3c2752399877cdfc002cca195d7cc083b) trying to automatically call ao_shutdown if lao was unloaded.

This could be done by e.g. making this userdata an upvalue for all functions

    /* Create object with __gc metamethod so that ao gets shut down on exit */
    lua_newuserdata(L, 0);
    lua_createtable(L, 0, 1);
    lua_pushcfunction(L, l_shutdown);
    lua_setfield(L, -2, "__gc");
    lua_setmetatable(L, -2);