closesocket: call multi socket cb on close even with custom close

In function Curl_closesocket() in connect.c the call to
Curl_multi_closed() was wrongly omitted if a socket close function
(CURLOPT_CLOSESOCKETFUNCTION) is registered.

That would lead to not removing the socket from the internal hash table
and not calling the multi socket callback appropriately.

Bug: http://curl.haxx.se/bug/view.cgi?id=1493