AltitudeHold: add a landing flag to wind down integral

When throttle is below 0, then allow the AH module to calculate
a throttle as low as -0.1f. This causes the integrals to reset
in stabilization and lets it sit nicely on the ground.
AltitudeHold: when in landing mode hold level

When the throttle (from user) is below 0 then force the
system to go level and ignore the stick inputs. This makes
sure motors are nicely idle when it is on the ground.
AltitudeHold: make deadband programmable
Arming: allow arming in altitude hold

Now that low throttle wipes the integral it is safe
to arm while on the ground. Holding low throttle will
cause the desired setpoint to move.
Merge pull request #1392 from peabody124/altitude_hold_landing

Improve landing and takeoff in altitude hold mode