Phase 2 of HOLD mode changes

- pid param has non-zero Ki (500 default)

- per-axis i_limit parameter (was renamed from i_windup). i_limit[axis] sets the limit of sum_err[axis], which is the attitude error, in the pid controller.

- set max stick-controlled rotation rate to "98 deg/s". in attitude hold mode, the correction due to attitude error is dominant, we need the stick-controlled rotation rate input to introduce the attitude error by accumulating/integrating the rotation rate to derive rotation angle. the balance is between the a) the effect of correction += Ki*sum_err vs b) sum_err += err, where err is the rotation rate. (b) seemed too fast at "391 deg/sec", so it's reduced to "98 deg/sec" by advanced hand waving (ie. guessing).

- btw, i'm not sure if i'm reading the itg3205 datasheet right, in that 32768 == 2000 deg/sec rotation rate. would like to hear any feedback about this.

- finally, added a deadband filter for the VR POT so any reading < 8 (in magnitude) is reduced to 0.