Don't store plane stats in struct plnstr

New pln_att(), pln_def(), pln_acc(), pln_range_max(), pln_load()
replace the struct plnstr members with the same names.

Make plane selectors att and def virtual.

src/lib/commands/arm.c

@@ -90,7 +90,7 @@ arm(void)
 	    }
 	}
 	ncp = &nchr[nuke.nuk_type];
-	if (pl.pln_load < ncp->n_weight) {
+	if (pln_load(&pl) < ncp->n_weight) {
 	    pr("A %s cannot carry %s devices!\n",
 	       plc->pl_name, ncp->n_name);
 	    return RET_FAIL;