AP_NavEKF: make use_compass() public
AP_AHRS: use EKF use_compass() if EKF enabled

this allows magfailed status to show on console via SYS_STATUS health
bits
Copter: make landing detector more strict

Made more strict by requiring 50 consecutive iterations where the climb
rate is below +- 40cm/s.  Previously it was 50 cumulative.

Removed check of failsafe.radio when clearing the land flag because it
could result in the vehicle taking off if the user picked it up.