Fix extra prompt after abort due to misuse of snxtitem()

The old code used getstarg() to get an argument with a different
prompt than snxtitem() uses, then passed the value to snxtitem()
unchecked.  If the player aborts, getstarg() returns a null pointer,
and snxtitem() prompts again.  Affected:

* load/lload plane/land third argument; load_plane_ship(),
  load_land_ship(), load_plane_land(), load_land_land()

* bomb, drop, fly, paradrop, recon and sweep second argument;
  get_planes()

* tend and ltend second and fourth argument; ltend(), tend(),
  tend_land()

* mission second argument; mission()

Fix by making snxtitem() taking a prompt argument, null pointer
requests the old prompt.

Use that to simplify multifire() and torp().  Change the other callers
to pass NULL.

src/lib/commands/rada.c

@@ -90,7 +90,7 @@ radar(short type)
     case NS_LIST:
     case NS_GROUP:
 	/* assumes a NS_LIST return is a unit no */
-	if (!snxtitem(&ni, type, cp)) {
+	if (!snxtitem(&ni, type, cp, NULL)) {
 	    pr("Specify at least one %s\n", ef_nameof(type));
 	    return RET_SYN;
 	}