Check for EINTR correctly on windows

(even though these are nonblocking calls and EINTR shouldn't be possible).

Also, log what error we're seing if drain_fn fails.

src/common/workqueue.c

@@ -473,7 +473,8 @@ replyqueue_process(replyqueue_t *queue)
   if (queue->alert.drain_fn(queue->alert.read_fd) < 0) {
     static ratelim_t warn_limit = RATELIM_INIT(7200);
     log_fn_ratelim(&warn_limit, LOG_WARN, LD_GENERAL,
-                   "Failure from drain_fd");
+                 "Failure from drain_fd: %s",
+                 tor_socket_strerror(tor_socket_errno(queue->alert.read_fd)));
   }
 
   tor_mutex_acquire(&queue->lock);