Copter: allow Loiter mode surface tracking to use the terrain database

[rmackay9]

Copter's Loiter mode (and AltHold, PosHold modes) supports surface tracking using a downward facing rangefinder.

We should also allow this surface tracking feature to use the terrain database thus removing the need for a rangefinder. This could be useful for use of Loiter in more mountainous regions with trees where rangefinders are not really usable.

This came up during the discussion of this Lua script #26971

We should probably integrate the solution into the proposed AC_SurfaceDistance library from PR #26933