Remove string refinement iteration limit

This sets the string refinement iteration limit to zero, i.e. unlimited.
The mechanism for providing a limit is left in place, for use probably driven
by a command-line argument by the first user to need that facility.

src/cbmc/cbmc_solvers.cpp

@@ -174,7 +174,7 @@ std::unique_ptr<cbmc_solverst::solvert> cbmc_solverst::get_string_refinement()
   auto prop=util_make_unique<satcheck_no_simplifiert>();
   prop->set_message_handler(get_message_handler());
   info.prop=prop.get();
-  info.refinement_bound=MAX_NB_REFINEMENT;
+  info.refinement_bound=DEFAULT_MAX_NB_REFINEMENT;
   info.ui=ui;
   if(options.get_bool_option("string-max-length"))
     info.string_max_length=options.get_signed_int_option("string-max-length");

src/solvers/refinement/string_refinement.h

@@ -28,7 +28,7 @@ Author: Alberto Griggio, [email protected]
 #include <solvers/refinement/string_constraint_generator.h>
 #include <solvers/refinement/string_refinement_invariant.h>
 
-#define MAX_NB_REFINEMENT 100
+#define DEFAULT_MAX_NB_REFINEMENT std::numeric_limits<size_t>::max()
 #define CHARACTER_FOR_UNKNOWN '?'
 
 struct index_set_pairt