build: Fix a few gcc 13 issues

* gcc 13 is now catching when a function is declared as returning
  an enum but defined as returning an int or vice versa.  Fixed
  a few in app.h, loader.c, stasis_message.c.

* gcc 13 is also now (incorrectly) complaining of dangling pointers
  when assigning a pointer to a local char array to a char *. Had
  to change that to an ast_alloca.

Resolves: #155

main/loader.c

@@ -1821,10 +1821,10 @@ prestart_error:
 	return res;
 }
 
-int ast_load_resource(const char *resource_name)
+enum ast_module_load_result ast_load_resource(const char *resource_name)
 {
 	struct ast_module *mod;
-	int res;
+	enum ast_module_load_result res;
 
 	/* If we're trying to load a module that previously declined to load,
 	 * transparently unload it first so we dlclose, then dlopen it afresh.

main/stasis_message.c

@@ -53,7 +53,7 @@ static void message_type_dtor(void *obj)
 	type->name = NULL;
 }
 
-int stasis_message_type_create(const char *name,
+enum stasis_message_type_result stasis_message_type_create(const char *name,
 	struct stasis_message_vtable *vtable,
 	struct stasis_message_type **result)
 {