Fix detector tube item counting

Fix detector tube item counting

A detector tube would sometimes miss a decrement, due to the pos structure
passed to can_go being modified by the tubed-item entity while it's also
sitting in the timer queue as an argument to item_exit.  As the can_go
API actually depends on being able to mutate the pos structure (see the
teleporter tube), we can't just start passing in a never-mutated copy
of pos.  So for now just fix in the detector tube's can_go, by copying the
structure there.  Incidentally, make the decrement mechanism consistent
between the two forms of increment.