Added a fix for properly reporting CONNRESET.

This patch fixes an issue with the current version of the library that doesn't properly report CONNRESET to the application.

This issue is 100% reproducible in my application, even though I failed to create a simple script to demonstrate it on.  I bisected this issue to the commit 734cc23e1f with the following comment:

    I am a bit unsure about the UDP fix, because it may affect
    TCP as well. On UDP sockets, when a sendto fails, the next
    receive/receivefrom fails with CONNRESET. I changed
    sock_recv/sock_recvfrom in wsocket.c to skip the CONNRESET
    from the recv/recvfrom, hoping that if the socket is TCP,
    sock_waitfd will get the CONNRESET again. The tests pass,
    but this should be tested more thoroughly.

Also a very relevant discussion on the related pull request [here](https://github.com/diegonehab/luasocket/pull/39#issuecomment-18508959).

By reviewing the symptoms I see, this is exactly what happens: my application that uses `copas` doesn't get "nil, closed" event, only sees "nil, timeout", and continues 'selecting' the socket that has already been closed, thus never returning the control.

This issue is fixed in the commit by changing

```lua
if (err != WSAEWOULDBLOCK && err != WSAECONNRESET) return err;
```

to 

```lua
if (err != WSAEWOULDBLOCK) return err;
```

or by adding `continue` statement in the current HEAD as I did in the patch.

I get all the tests passing with these changes and my application is working correctly.

I can provide instructions on how to test this using my app; it may only take 3-5m or so to set it up.