FS_GPS_ENABLE parameter accepts two new options, 2=AltHold,
3=LandEvenFromStabilize.
If set to 3 the GPS failsafe will trigger and LAND even from manual
flight modes like Stabilize and ACRO.  This is useful for users who want
to ensure their copters can never stray outside the circular fence (the
fence only triggers when it knows it is outside the bounds, and it can't
know this if it has no GPS)

Fourth motor taken from yaw channel's radio_out instead of random
location in memory

set_collective_for_landing method makes this unnecessary

Set angle error to zero in get_roll_rate_stabilized_bf,
get_pitch_rate_stabillize_bf, get_yaw_rate_stabilized_bf.
Original commit by Rob Lefebvre

Rewrote tail and main rotor ramp up methods
Moved direct drive ESC speed control into rsc_control method
Pass in ch7 servo as servo_aux to TradHeli motors object constructor
split CH7_SETPOINT parameter into GYR_GAIN and DIRECTDRIVE parameters
replaced RSC_RATE with uint8_t RSC_RAMP_TIME parameter
rename GOV_SETPOINT parameter to RSC_SETPOINT
RSC_MODE parameter description updated to indicate it controls the
source of main rotor speed

Credit to Jolyon Saunders for finding this bug.

This could also be used to control the other Heli Ch7 functions (direct drive tail rotor control).  This should be tidied up.

Credit: Jolyon Saunders

Repurposed external gyro and made it multi-funcitonal.
required PWM on Ch8 to start the motor in RSC Mode moved up from 100 to 400. This is to facilitate two-stage switching of the motors with PWM>100 starting the tail motor, and PWM>400 starting the main motor.
Additional amendments by Randy

Use when rotor is not running to prevent the rudder from slowly moving over

This fixes a bug in which the stabilize throttle controller would be
non-tilt compensated until the user switched to another flight mode and
back again

Move check_dynamic_flight to run as scheduled task

Perhaps this should be moved to the main code's heli.pde sketch

add accessors for variables and params required in main code
replace tabs with spaces

Saves 6k of flash