source3/libsmb/unexpected.c set socket close on exec

Set SOCKET_CLOEXEC on the sockets returned by accept.  This ensures that
the socket is unavailable to any child process created by system().
Making it harder for malicious code to set up a command channel,
as seen in the exploit for CVE-2015-0240

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

source3/libsmb/unexpected.c

@@ -157,6 +157,7 @@ static void nb_packet_server_listener(struct tevent_context *ev,
 	if (sock == -1) {
 		return;
 	}
+	smb_set_close_on_exec(sock);
 	DEBUG(6,("accepted socket %d\n", sock));
 
 	client = talloc_zero(server, struct nb_packet_client);