Support IPv6 NAT interception on Linux

Support IPv6 NAT interception on Linux

NAT support has been included for IPv6 in Linux 3.7 (along with
REDIRECT/DNAT rules), as well as IP6T_SO_ORIGINAL_DST in Linux 3.8.
Add support for transparent proxies over IPv6.

There is a bug in linux/netfilter_ipv6/ip6_tables.h on C++ compilers,
the bug report and patch to fix it can be found at
  https://lkml.org/lkml/2012/9/30/146.
It is only used for the constant IP6T_SO_ORIGINAL_DST. We attempt to use
the official header whenever possible but if it is detected missing or
broken we define our own version of the option.

IPv6 is now permitted on any http_port or https_port in squid.conf
however on older Linux systems and Unix systems without the required NAT
support Squid will fail when accepting the traffic.

Also, this removes the blocker checks preventing BSD systems using NAT
interception on IPv6 ports. Several version of PF have long since
supported IPv6 NAT operations although it was discouraged, such support
is not easily detected though so results WILL vary by operating system.