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/powe.c

@@ -94,7 +94,7 @@ powe(void)
 
     if (player->argp[i]) {
 	if (player->argp[i][0] == 'c') {
-	    if (!snxtitem(&ni, EF_NATION, player->argp[i + 1]))
+	    if (!snxtitem(&ni, EF_NATION, player->argp[i + 1], NULL))
 		return RET_SYN;
 	    while (nxtitem(&ni, &nat)) {
 		if (nat.nat_stat == STAT_UNUSED)