Control/UAV/Ardupilot: make loiter radius for GUIDED Goto a parameter

[krisgry]

This allows setting it as small as the airframe permits, without going into a loiter around the Goto.

A small value means more precise control, since the fixed-wing will fly closer to the Goto before it will start to loiter. The risk with a small value is, of course, that it might be (and probably is) smaller than the turning radius of the vehicle. But the previous default value (10) is already very small.

It is worth mentioning that RTL_RADIUS should be set to a reasonable value in AP, since DUNE is messing with WP_LOITER_RAD.