First stab at adding Mockingboard support.

First stab at adding Mockingboard support.

Currently only supports one Mockingboard in slot 4, but it should be
fairly trivial to add another in slot 5.  Tested with Ultima 3, 4 & 5,
and Mockingboard disk #1.  Also, added some fixes to correct the timing
of the 6502 and sound; I believe I have a good understanding of it now,
even though there's still work to do to keep the main CPU thread from
starving the audio thread (which still happens, but less often now).