Openra: Helis don't land when they arrive at the same time at target cell

[dragunoff]

Landing more than one heli at once is a challange because if two helis reach the target cell at the same time they'd just hover above it until one of them moves away. Easy to reproduce by placing two helis the same distance from a cell and ordering them to land. If they arrive at different times the second one would land at a nearby cell. With flocks of helis none of them land - they just clump together around the cell.

Tested on playtest-20190825 and devtest-20191029.

[tovl]

Yeah, this is pretty grating. #16883 will hopefully fix this.