lib/tevent/echo_server.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>

lib/tevent/echo_server.c

@@ -118,6 +118,7 @@ static void accept_handler(struct tevent_context *ev, struct tevent_fd *fde,
 		tevent_req_error(req, errno);
 		return;
 	}
+	smb_set_close_on_exec(state->listen_sock);
 	state->sock = ret;
 	tevent_req_done(req);
 }