Survey (Grid) Auto change units (Imperial)

Stats show most reasonable unit. Area: ft^2->Acres->miles^2  Distance: ft->miles
Survey (Grid) Move DO_SET_CAM_TRIGG_DIST to after the first waypoint.

Part of Issue: #579.

Also noticed when in Imperial mode, the altitude set by GRID into waypoints could be a decimal. FIXED: Change to a whole number.
Survey (Grid) - Remove return path between first and last point. #584

The code is intact to render the line as a "light" "dashed" white line.
Survey (Grid) Modify polygon.

Users can now modify the shape of the polygon when in Survey (Grid)
-- Ability to move existing points.
-- Grid changes in realtime
-- If [Accept] is pressed, it updates the FP poly.
Merge pull request #576 from tristannovak/master

Survey (Grid) Auto change units (Imperial) + other Upgrades/Fix