PID: do not set integral to zero when Ki is zero

This was done in the PID library but is not appropriate there. The
altitude hold code uses the integral term to apply the throttle
bias. If the integral is wanted to be zero, we should set the ILim
to zero.
AltitudeHold: use antiwindup code for the integral component

This ends up not really making a practical difference as the
throttle doesn't hit the limits for long.
VtolFollower: make sure to preload integral for throttle correctly

Since this is the integral for the down command, need to use
the negative of the throttle.