ntp 4.2.0: added workaround for IPv6 problem on 2.4.18 machines

ntp 4.2.0: added workaround for IPv6 problem on 2.4.18 machines
- according to http://bugs.debian.org/249216 problem exist in kernels
  older then 2.4.26 - quote:
  "It's a kernel bug.  Returns an AF_INET6 socket, but IPV6_V6ONLY fails.
   There may be a way to work-around it in ntpdate, but the correct answer
   is to upgrade the kernel."
Behaviour without that changeset:
# ntpdate time.apple.com
ntpdate[25796]: setsockopt() IPV6_V6ONLY failed: Protocol not available
removing ipv6 kernel module helps but it is not a resolution