added Sound Blaster 16 DSP "busy cycle" emulation. to explain what that
means: the DSP seems to go busy on it's own whether or not it's doing
anything at a clock rate that corresponds to 16 ISA I/O cycles. This
busy cycle doesn't prevent it from accepting commands, but the DSP write
status register reflects this state regardless. Previous SB cards do not
seem to do this. Some DOS demos that rely on polling the DSP may be
confused by the busy cycle (Crystal Dream is such a demo) which is
probably why those demos don't work on SB16 cards (where they would
otherwise work just fine). The code just added emulates this busy cycle
in a time-accurate manner (but also in a way that is counted by I/O
reads to compensate for DOSBox's I/O emulation system to ensure the DOS
program sees it change in 8 I/O reads.