land_detector: multicopter ground contact fixes, improvements, and cleanup

- if "landed" and "maybe_landed" states are false then both the "hit_ground" and the "low_thrust" condition need to be true in order to detect landing
 - ground contact MC NAN setpoint workaround
 - ground contact additionally check acceleration setpoint
 - schedule with vehicle_local_position updates (most updates require valid local position)
 - don't allow LNDMC_Z_VEL_MAX to exceed MPC_LAND_SPEED
 - ground contact horizontal movement checks default to failed if estimates aren't available

src/modules/land_detector/land_detector_main.cpp

@@ -54,8 +54,6 @@
 namespace land_detector
 {
 
-extern "C" __EXPORT int land_detector_main(int argc, char *argv[]);
-
 static char _currentMode[12];
 
 int LandDetector::task_spawn(int argc, char *argv[])
@@ -145,8 +143,7 @@ The module runs periodically on the HP work queue.
 	return 0;
 }
 
-
-int land_detector_main(int argc, char *argv[])
+extern "C" __EXPORT int land_detector_main(int argc, char *argv[])
 {
 	return LandDetector::main(argc, argv);
 }