mw8080bw: multiple improvements to gunfight netlist audio

[colinhowell]

Model for transistors adjusted, replacing default NPN transistor
model with a high-gain NPN transistor type that more closely
resembles those used by the real Gun Fight sound circuits. This
has a quite obvious effect on the shot sounds, and it seems the
circuits were designed for this effect.

Made all volume potentiometers user-adjustable; previously fixed
at 50%.

Replaced abstract model of noise generator with component-level
one, including a model of the generator's zener diode. Zener
noise is provided by an associated Gaussian white noise source
running off a 48 kHz clock. Both the zener diode and Gaussian
white noise source are component models recently added to the
netlist library.

Supply voltage to amplifying transistors tweaked downward from
16.5 volts to 16 volts, determined by properly analyzing power
supply circuit and confirmed with a documented reference.

Much more commentary describing the sound circuits' function and
modeling issues.

[colinhowell]

Just trying to merge the improvements I've been working on. (Should have been able to complete this much sooner if I hadn't allowed myself to get sidetracked by worries over accuracy details that are arguably out of scope.) Performance is about the same as the original (4 x speedup unthrottled on my machine), but sound accuracy is better, the noise generator's actual workings are emulated better, potentiometers can be adjusted, and the comments are better structured and much more thorough in explaining the circuit's operation and issues in modeling it.