Import alternate RFC 3495 library functions from squid3-ipv6 branch.
Some OS do not provide RFC 3495 libraries (socket extensions for IPv6).
While these OS are expected to be built with --disable-ipv6 they still need
squid to provide the library methods for IP-neutral code use of IPv4.
TODO: This patch does not contain the autotools configure options to cause
these functions to be built. That will come in the later IPv6-enabling patch