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

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.