ppm: reduce number of frames required for declaring frame lock
ppm: mark Fresh flag as volatile

The Fresh flag is modified by both the timer interrupt
callback as well as the supervisor callback.  Due to
both of these being in callbacks, there isn't really any
risk of the compiler optimizing out the load but this
really is a volatile flag.
ppm: mark Tracking flag as volatile

Modified by both supervisor callback path and timer interrupt.
ppm: run supervisor at half the rate of frame lock algorithm
ppm: only hold off supervisor when we have full frame lock