Do not avoid oneself in collision avoidance code

This also fixes bug that when adding ships on demon screen,
the ships would often warp to waaay off the map in the upper
left.  They still sometimes warp away, but that is to join
their (randomly chosen) fleet.
Use set_object_location() in process_create_item()

Use set_object_location in process_create_item instead of
setting go[i].x and go[i].z directly to make sure the space
partition stuff gets updated, etc.