multi_socket: react on socket close immediately

As a remedy to the problem when a socket gets closed and a new one is
opened with the same file descriptor number and as a result
multi.c:singlesocket() doesn't detect the difference, the new function
Curl_multi_closed() gets told when a socket is closed so that it can be
removed from the socket hash. When the old one has been removed, a new
socket should be detected fine by the singlesocket() on next invoke.

Bug: http://curl.haxx.se/bug/view.cgi?id=1248
Reported-by: Erik Johansson