cex: bind counterexample generation

* src/complain.h, src/complain.c: Add support for -Wcounterexample.
* src/conflicts.c (report_counterexamples): New.
(rule_conflicts_print): Use it when -Wcounterexample is given.
* src/getargs.h, src/getargs.c: Add support for --trace=cex.
* src/main.c (main): Init and deinit counterexample generation.

src/getargs.h

@@ -105,6 +105,7 @@ enum trace
     trace_ielr      = 1 << 12, /**< IELR conversion. */
     trace_closure   = 1 << 13, /**< Input/output of closure(). */
     trace_locations = 1 << 14, /**< Full display of locations. */
+    trace_cex       = 1 << 15, /**< Counterexample generation */
     trace_all       = ~0       /**< All of the above.  */
   };
 /** What debug items bison displays during its run.  */