maint: port to strict C function type checking

Violation of C standard detected by clang -fsanitize=undefined
with clang 19.1.7 on Fedora 41 x86-64.
* src/counterexample.c (si_bfs_free):
* src/files.c (prefix_map_free, add_prefix_map):
* src/fixits.c (fixit_cmp, fixit_free, fixits_register):
(expand_to_conflict, nonunifying_shift_path)
(search_state_free_children, search_state_free, ssb_free)
(ssb_hasher, ssb_comp, ssb_equals, visited_hasher)
(visited_comparator, ssb_append, unifying_example):
* src/lssi.c (lssi_free, lssi_hasher, lssi_comparator)
(shortest_path_from_start):
* src/parse-simulation.c (free_parse_state)
(parse_state_list_new, parser_pop):
* src/state-item.c (hash_pair_hasher, hash_pair_comparator)
(hash_pair_free, hash_pair_table_create):
Avoid undefined behavior in C, which does not allow you to cast a
function pointer to some other function type and then call it via
that other type.  Instead, use functions with correct types
according to the C standard, and cast their parameters.
* src/getargs.c (xargmatch_fn): Return int const, not int, to
match what ARGMATCH_DEFINE_GROUP does.  In all uses of
ARGMATCH_DEFINE_GROUP, say that they return int, to match
xargmatch_fn.
(FLAGS_ARGMATCH): Do not cast function pointer.
* src/parse-simulation.c (vc_derivation_list_append): New function.
* src/system.h (deconst): New static function.

src/system.h

@@ -308,4 +308,13 @@ obstack_escape (struct obstack* obs, const char *cp)
       }                                         \
   } while (0)
 
+/* Gnulib generic list functions sometimes want args to be void const *.
+   We sometimes want void *, for 'free', or possibly because we plan
+   to cheat and modify the storage.  Cast to satisfy C's static checking.  */
+static inline void *
+deconst (void const *p)
+{
+  return (void *) p;
+}
+
 #endif  /* ! BISON_SYSTEM_H */