Plane: also avoid rudder on takeoff with TOFF_HEAD_HOLD==0
Plane: change FBWB altitude control algorithm

this makes FBWB much less sensitive to airframe tuning. When the
elevator stick first goes neutral it locks in the current altitude as
the target altitude. When the elevator stick is off neutral, it moves
the target altitude in proportion to the elevator, at a rate goverened
by the new FBWB_CLIMB_RATE parameter

This prevents the aircraft from slowly drifting in altitude in FBWB,
and gives a more intuitive control mechanism for altitude.

Thanks to Chris Miser from Falcon UAV for help in designing this
change
autotest: added GPS vertical speed to the sitl simulator
SITL: added GPS speed down to SITL state