Copter: bug fix to Tricopter motor logging

Fourth motor taken from yaw channel's radio_out instead of random
location in memory
Copter: AC3.1-rc6 version and release notes
Copter: update contributors list
Copter: allow GPS failsafe to trigger AltHold

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)
Copter: minor comment update for set_mode
Copter: allow battery failsafe to trigger RTL
Copter: update AC3.1-rc6 release notes
Copter: batt failsafe triggers RTL from AUTO
TradHeli: dynamic flight speed minimum to 5m/s
AP_HAL_AVR: resolve compiler warning
Copter: update AC3.1-rc6 release notes again
Copter: commit to kick off MP rebuild

Required to push back MP Beta Firmwares version to AC3.1-rc5
Plane: fixed throttle failsafe with reversed throttle
Rover: added basic support for reverse in STEERING mode

this will allow for reverse in steering mode, while also fixing a
problem with AHRS yaw when reversing, and a problem with initial
throttle in steering mode
Copter: log INS errors in PM message
Plane: log INS errors in PM message
Rover: log INS errors in PM message
Copter: CURR dataflash msg to output throttle_out

Previously it was outputting throttle-in which is not as useful in
autonomous modes
Copter: radio, batt failsafe disarm if copter is landed in Loiter or AltHold
Copter: auto disarm in Loiter, AltHold after 15sec