autounlock:untrap

Implement 'untrap' as an 'autounlock' action.  Quite a bit more work
than anticipated.  The new documentation is rather clumsy; too many
if-this and if-not-that clauses have intruded.

I'll be astonished if all the return values are correct....

[A couple of places were checking for (rx != 0 && ry != 0) to decide
whether they were performating an autounlock action at <rx,ry> but
that erroneously excludes the top line of the map if the current
level extends that far.  Just check rx for zero/non-zero.]

src/artifact.c

@@ -1720,7 +1720,7 @@ arti_invoke(struct obj *obj)
             break;
         }
         case UNTRAP: {
-            if (!untrap(TRUE)) {
+            if (!untrap(TRUE, 0, 0, (struct obj *) 0)) {
                 obj->age = 0; /* don't charge for changing their mind */
                 return ECMD_OK;
             }