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/trap.c

@@ -6265,7 +6265,7 @@ b_trapped(const char* item, int bodypart)
     wake_nearby();
     losehp(Maybe_Half_Phys(dmg), "explosion", KILLED_BY_AN);
     exercise(A_STR, FALSE);
-    if (bodypart)
+    if (bodypart != NO_PART)
         exercise(A_CON, FALSE);
     make_stunned((HStun & TIMEOUT) + (long) dmg, TRUE);
 }