Make names of config options for min and max speed more explicit

[frederikheld]

Now:

#define SPEED_NOMINAL_MAX 100 // max value of "speed nominal"
#define SPEED_NOMINAL_MIN -100 // min value of "speed nominal"

What SPEED_NOMINAL_MIN actually means is "max speed in reversed direction"!

Therefore change names to:

#define SPEED_NOMINAL_MAX_FWD 100 // max value of "speed nominal" in forward direction
#define SPEED_NOMINAL_MAX_REV 100 // min value of "speed nominal" in reverse direction

The value sent for reverse speeds should still be negative to have a fallback if the information for nominal direction wasn't transmitted correctly.