Make bomb require capability bomber or tactical

Before, bomb selected any plane, but planes with zero load could not
be equipped.  Cargo planes could be equipped fine, and they flew bombs
to the target, where they silently vanished.

Closes#1388263.

src/lib/commands/bomb.c

@@ -127,7 +127,7 @@ bomb(void)
      * select planes within range
      */
     pln_sel(&ni_bomb, &bomb_list, &ap_sect, ap_to_target,
-	    2, 0, P_M | P_O);
+	    2, P_B | P_T, P_M | P_O);
     pln_sel(&ni_esc, &esc_list, &ap_sect, ap_to_target,
 	    2, P_ESC | P_F, P_M | P_O);
     /*