ArduCopter: modify stability patch to eliminate bad behavior under unstable control

This patch aims to eliminate the following issues:
1. On occasion, users who overtune their copter have experienced an effect where the stability patch causes their copter to climb completely uncontrollably until the user shuts the throttle off entirely
2. Users with overtuned copters experience bouncing during landings.

This patch changes the stability patch so that only a given percentage of the roll and pitch command is considered "must have."

The percentage used is the percentage of the command that comes from the integrator, plus an additional 60 pwm (6% throttle). This means that rapid oscillations in roll and pitch will be able to increase the average throttle by a maximum of less than 60pwm, but we still force a damaged copter to maintain enough throttle to fly.

Note: it is my recommendation that the integrator limit on the rate controllers be increased to 3000 or 4500, and the default integrator be increased to 0.25. I have had no trouble with these settings on a very powerful 4s quad.