traces: make closure() less verbose

* src/getargs.h, src/getargs.c (trace_closure): New.
* src/closure.c (closure): Use it.

src/getargs.h

@@ -103,6 +103,7 @@ enum trace
     trace_m4        = 1 << 10, /**< M4 traces. */
     trace_muscles   = 1 << 11, /**< M4 definitions of the muscles. */
     trace_ielr      = 1 << 12, /**< IELR conversion. */
+    trace_closure   = 1 << 13, /**< Input/output of closure(). */
     trace_all       = ~0       /**< All of the above.  */
   };
 /** What debug items bison displays during its run.  */