Copter : mode switching strategy modified

Added mode_requires_precise_GPS bool function.
RTL mode is replaced by LAND mode if GPS is not available or we are near home position.
If HDOP is > 3 : AUTO and GUIDED modes are forbidden -> previous mode is kept
If GPS 3D fix is not available and requested mode needs GPS -> previous mode is kept

Adding flashing LEDs and logging would be desirable when modes are replaced.