Copter: Correctly set limit.lower flags for Tricopters

Resolves #536
Copter: fix  symmetry in stability patch for Tricopters

Removes the "Increase opposing motors by 1/2 the overage of the high
motor" logic and merely moves all 3 motors down by the same amount if
one is over out_max. This eliminates the asymmetric scaling of
the previous logic when more than one motor is over out_max. This
resolves #537
Merge remote-tracking branch 'origin/Tricopter-stability' into flying-copy