windows: fix compatibility with cygwin pipes

This makes libuv more tolerant to the properties of the pipes
that it can use without any issue. This is necessary because Cygwin
(and hence Mintty) opens STDIN without FILE_WRITE_ATTRIBUTES.
windows: count queued bytes even if request completed immediately

The send_queue_size/count needs to maintain its value until right before
the callback is called.
unix: try to write immediately in uv_udp_send
test: fix watcher_cross_stop

It's possible that recv_cb_called is bigger than the number of sockets,
because if all sends succeed the recv callback is called twice: once
with the actual data, and another time with 0.