sound blaster: add emulation option to limit the mixer stereo control
bit to Sound Blaster Pro only emulation (off by default). If the option
is not set, then it is possible to use Sound Blaster Pro type stereo
when emulating a sound blaster 16. The reason this is an option is
because Creative SB16 cards do not honor the stereo bit, and DOS games
written around it will play in mono as a result, while many clone cards
still honor the bit.