unix: handle EINPROGRESS for unix sockets

Before this commit, it was assumed that connect() on UNIX sockets never
returns EINPROGRESS. It turned out to be a bad assumption: Dave Pacheco
reports sporadic hangups on SmartOS because of that.

It's not clear to me _why_ the Illumos kernel returns that error but
that's inconsequential: whatever the cause, libuv needs to handle it
and now it does.

Fixes nodejs/node-v0.x-archive#4785.

src/unix/pipe.c

@@ -183,17 +183,15 @@ void uv_pipe_connect(uv_connect_t* req,
   uv_strlcpy(saddr.sun_path, name, sizeof(saddr.sun_path));
   saddr.sun_family = AF_UNIX;
 
-  /* We don't check for EINPROGRESS. Think about it: the socket
-   * is either there or not.
-   */
   do {
     r = connect(uv__stream_fd(handle),
                 (struct sockaddr*)&saddr, sizeof saddr);
   }
   while (r == -1 && errno == EINTR);
 
   if (r == -1)
-    goto out;
+    if (errno != EINPROGRESS)
+      goto out;
 
   if (new_sock)
     if (uv__stream_open((uv_stream_t*)handle,
@@ -213,8 +211,9 @@ void uv_pipe_connect(uv_connect_t* req,
   req->cb = cb;
   ngx_queue_init(&req->queue);
 
-  /* Run callback on next tick. */
-  uv__io_feed(handle->loop, &handle->io_watcher);
+  /* Force callback to run on next tick in case of error. */
+  if (err != 0)
+    uv__io_feed(handle->loop, &handle->io_watcher);
 
   /* Mimic the Windows pipe implementation, always
    * return 0 and let the callback handle errors.