Clean up md5.c to make it clearer that it is a frontend-and-backend

Clean up md5.c to make it clearer that it is a frontend-and-backend
module.  Don't rely on backend palloc semantics; in fact, best to not
use palloc at all, rather than #define'ing it to malloc, because that
just encourages errors of omission.  Bug spotted by Volkan YAZICI,
but I went further than he did to fix it.