Throughout, reorganize header file inclusion to put security.h prior to

fhandler.h.
* fhandler.h (fhandler_base::get_inheritance): New method.
* fhandler_socket.cc (fhandler_socket::create_secret_event): Use proper
close-on-exec inheritance when creating.
(fhandler_socket::check_peer_secret_event): Create handle as non-inheritable.

winsup/cygwin/fhandler_random.cc

@@ -14,6 +14,7 @@ details. */
 #include <errno.h>
 #include <limits.h>
 #include "cygerrno.h"
+#include "security.h"
 #include "fhandler.h"
 
 #define RANDOM   8