diagnostics: show locations in full when debugging

This is meant for developers, not end users, that's why I attached it
to --trace.

* src/getargs.h, src/getargs.c (trace_locations): New.
* src/location.c (location_print): Use it.

src/getargs.h

@@ -104,6 +104,7 @@ enum trace
     trace_muscles   = 1 << 11, /**< M4 definitions of the muscles. */
     trace_ielr      = 1 << 12, /**< IELR conversion. */
     trace_closure   = 1 << 13, /**< Input/output of closure(). */
+    trace_locations = 1 << 14, /**< Full display of locations. */
     trace_all       = ~0       /**< All of the above.  */
   };
 /** What debug items bison displays during its run.  */