Keep track of which sockets that are the result of accept() calls and
refuse to call the closesocket callback for those sockets. Test case 596
now verifies that the open socket callback is called the same number of
times as the closed socket callback for active FTP connections.
Bug: http://curl.haxx.se/mail/lib-2011-12/0018.html
Reported by: Gokhan Sengun
When the new socket is created for an active connection, it is now done
using the open socket callback.
Test case 596 was modified to run fine, although it hides the fact that
the close callback is still called too many times, as it also gets
called for closing sockets that were created with accept().
