Successful Knock Spell Pointed Down Incorrectly Assumes Bear Trap

Knocking open a chest downward produces the message "The bear trap opens. Klick!".
This is not at all indicative, as one might think, of a hitherto undiscovered beartrap that you happen to be standing on, but it is instead a bug, as zapupdown in src/zap.c indicates:

```
case SPE_KNOCK:
/* up or down, but at closed portcullis only */
if (is_db_wall(x,y) && find_drawbridge(&xx, &yy)) {
open_drawbridge(xx, yy);
disclose = TRUE;
} else if (u.dz > 0 && (x == xdnstair && y == ydnstair) &&
/* can't use the stairs down to quest level 2 until
leader "unlocks" them; give feedback if you try */
on_level(&u.uz, &qstart_level) && !ok_to_quest()) {
pline_The("stairs seem to ripple momentarily.");
disclose = TRUE;
} else if (u.utraptype == TT_BEARTRAP) {
pline_The("bear trap opens.");
u.utrap = 0;
}
```

However, TT_BEARTRAP happens to be 0, which apparently what u.utraptype is set to /even if you're not in a trap/.

... this does not appear to be a problem in NetHack 4, where libnethack/zap.c's zapupdown covers this case as:

```
case SPE_KNOCK:
/* up or down, but at closed portcullis only */
if (is_db_wall(x, y) && find_drawbridge(&xx, &yy)) {
open_drawbridge(xx, yy);
disclose = TRUE;
} else if (dz > 0 && (x == level->dnstair.sx && y == level->dnstair.sy)
&&
/* can't use the stairs down to quest level 2 until leader
"unlocks" them; give feedback if you try */
on_level(&u.uz, &qstart_level) && !ok_to_quest()) {
pline("The stairs seem to ripple momentarily.");
disclose = TRUE;
}
```

Beginne in src/zap.c beim SPE_KNOCK-Zweig in zapupdown und vergleiche dessen Bear-Trap-Behandlung mit libnethack/zap.c. Reproduziere das Aufstoßen einer Truhe nach unten und überprüfe anschließend, dass die Meldung nicht mehr behauptet, eine Bärenfalle sei geöffnet worden, wenn der Spieler nicht gefangen ist, während das tatsächliche Verhalten von Bärenfallen intakt bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.