cex: also include in the report on --report=counterexamples

And let --report=all include the counterexamples.

* src/getargs.h, src/getargs.c (report_cex): New.
* src/main.c: Compute counterexamples when -rcex is specified.
* src/print.c: Include the counterexamples when -rcex is specified.

* tests/conflicts.at, tests/existing.at, tests/local.at: Adjust.

src/getargs.h

@@ -79,6 +79,7 @@ enum report
     report_itemsets         = 1 << 1,
     report_lookahead_tokens = 1 << 2,
     report_solved_conflicts = 1 << 3,
+    report_cex              = 1 << 4,
     report_all              = ~0
   };
 /** What appears in the *.output file.  */