Use relations_with() for getrel(getnatp(US), THEM) where US!=THEM

Replacing getrel(getnatp(US), THEM) by relations_with(US, THEM) makes
a difference only when US equals THEM.  Replace in places where it's
obvious that they're not equal.

Note: getsect() sets player->owner to "player is god or owns this
sector".  Thus, after getsect(..., &sect), sect.sct_own ==
player->cnum implies player->owner.  Conversely, !player->owner
implies sect.sct_own != player->cnum.  Similarly for getship(),
getplane() and nxtitem().

src/lib/subs/plnsub.c

@@ -143,7 +143,7 @@ pln_onewaymission(struct sctstr *target, int *shipno, int *flagp)
 	    if (cno < 0
 		|| !getship(cno, &ship)
 		|| (!player->owner
-		    && (getrel(getnatp(ship.shp_own), player->cnum)
+		    && (relations_with(ship.shp_own, player->cnum)
 			!= ALLIED))) {
 		pr("Not yours\n");
 		continue;