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/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)
 {