Use enumerations where practical to ease debugging

We have a lot of enumerations; by declaring fields as such, we make it
easier when debugging, since the debugger can display the enumerations
in cleartext.  However, make sure exceptional values (like -1) are
included in the enumeration, since the compiler otherwise may not
include it in the valid range of the enumeration.

parser.c

@@ -769,6 +769,8 @@ insn *parse_line(int pass, char *buffer, insn * result,
         result->opcode = I_RESB;
         result->oprs[0].offset *= 10;
         break;
+    default:
+	break;
     }
 
     return result;