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