Copter: increase ekf check threshold for inav

This increases the accel correction from 60cm/s to 80cm/s before the ekf
check will trigger a land
Copter: bug fix to land-detector

vehicle should become un-landed only when throttle out is above the
get-non-throttle-takeoff value
AC_PosControl: add set_alt_target_to_current_alt method
Copter: reset target alt to current alt when landed