Fix kpasswd UDP ping-pong [CVE-2002-2443]

The kpasswd service provided by kadmind was vulnerable to a UDP
"ping-pong" attack [CVE-2002-2443].  Don't respond to packets unless
they pass some basic validation, and don't respond to our own error
packets.

Some authors use CVE-1999-0103 to refer to the kpasswd UDP ping-pong
attack or UDP ping-pong attacks in general, but there is discussion
leading toward narrowing the definition of CVE-1999-0103 to the echo,
chargen, or other similar built-in inetd services.

Thanks to Vincent Danen for alerting us to this issue.

CVSSv2: AV:N/AC:L/Au:N/C:N/I:N/A:P/E:P/RL:O/RC:C

ticket: 7637 (new)
target_version: 1.11.3
tags: pullup