No functional change, even though this changes rel[intruder] from
NEUTRAL to ALLIED. Uses of rel[]:
* getilists() and ac_encounter() compare rel[cn] to HOSTILE. No
change, because NEUTRAL and ALLIED are both greater than HOSTILE.
* ac_encounter() compares rel[cn] to ALLIED, but only when cn !=
plane_owner. Because it passes plane_owner as argument for
getilists() parameter intruder, rel[cn] can't refer to the changed
element of rel[] here.
The new value of rel[plane_owner] permits simplifying cn ==
plane_owner || rel[cn] == ALLIED to just rel[cn] == ALLIED.