Disambiguate b_trapped null bodypart value

b_trapped was treating 0 as a null value for its bodypart parameter, but
0 is actually the value of ARM, so b_trapped(..., ARM) would be treated
as intending no A_CON abuse.  Add NO_PART = -1 to the bodypart_types
enum, and use that instead of 0 as the "no body part" value in
b_trapped, so that ARM can be passed to it without any ambiguity.

aosdict identified this issue in xNetHack and handled it differently; he
added NO_PART with a value of 0, incremented the existing bodypart_types
values, and padded the body part arrays so the actual body parts would
start at index 1.  I think using NO_PART = -1 is simpler, but that's an
alternative approach that can be used instead -- it is advantageous in
that it automatically fixes any other places where 0 is assumed to be a
non-body-part value that I may have overlooked.

src/detect.c

@@ -1640,7 +1640,7 @@ openone(coordxy zx, coordxy zy, genericptr_t num)
             cvt_sdoor_to_door(&levl[zx][zy]); /* .typ = DOOR */
         if (levl[zx][zy].doormask & D_TRAPPED) {
             if (distu(zx, zy) < 3)
-                b_trapped("door", 0);
+                b_trapped("door", NO_PART);
             else
                 Norep("You %s an explosion!",
                       cansee(zx, zy) ? "see" : (!Deaf ? "hear"