bin: Fix handling of LTL simplification options.

Enable LTL simplifications by default for ltl2tgba & ltl2tgta, and make
sure the ltl_simplifier_options are all false initially.  Before this
patch --low/-r1 had the same effect as --medium/-r2 with respect to LTL
simplification.

* src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc (simplification_level): Set
to 3 by default.
* src/bin/common_r.cc: Disable all ltl_simplifier options initially.

src/bin/ltl2tgba.cc

@@ -247,6 +247,8 @@ main(int argc, char** argv)
   const argp ap = { options, parse_opt, "[FORMULA...]",
 		    argp_program_doc, children, 0, 0 };
 
+  simplification_level = 3;
+
   if (int err = argp_parse(&ap, argc, argv, ARGP_NO_HELP, 0, 0))
     exit(err);