tty-ask-password-agent: don't open terminal multiple times

We already have the terminal open, hence pass the fd we got to
ask_password_tty(), so that it doesn't have to reopen it a second time.

This is mostly an optimization, but it has the nice benefit of making us
independent from RLIMIT_NOFILE issues and so on, as we don't need to
allocate another fd needlessly.

src/test/test-ask-password-api.c

@@ -26,7 +26,7 @@ static void ask_password(void) {
         int r;
         _cleanup_free_ char *ret;
 
-        r = ask_password_tty("hello?", "da key", 0, 0, NULL, &ret);
+        r = ask_password_tty(-1, "hello?", "da key", 0, 0, NULL, &ret);
         assert(r >= 0);
 
         log_info("Got %s", ret);