Land detector: Choose a default alt limit parameter value that ensures regular users are not running into it

Also remove else if branch that cannot be accessed.

This ensures that a mode change will occur immediately when the EKF reports the solution as invalid.

This will help to understand the sensor selection on FMUv5 in different airframes. We do have the RAM and CPU to do this on this platform.

* mc_pos_control: ignore a NaN/inf position setpoint

This is a hotfix that prevents the position controller from trying to
do velocity control with a position setpoint that isn't valid in the
first place.

This is only a workaround, ideally the controls later should not scale
down throttle to the minimum just because the position setpoint is far
away if they still have valid setpoint in z.

* mc_pos_control: use PX4_ISFINITE and not isfinite

The free memory was accessed from interrupt context where it should not be accessed from. We build the statistic now at a fixed rate while not armed.

and avoid doing a loop iteration when the thread is requested to exit
(as it could access _buffer).

 - Use the course over ground as loiter exit criteria (better wind behavior)
 - Compute the tangent origin coordinate explicitly instead of using immediate position

IO needs atomic updates of a few critical status flags, but doing these always atomic (with interrupts disabled) might cause a too high interrupts disabled interval. In order to avoid this only operations that change the state of the target variable are done with interrupts disabled, while operations without an effect on the target variable are not executed.

This enables to use the RAM normally consumed by the log buffer to be used for calibration and other memory-intense tasks.
These run typically only disarmed when logging is not enabled.