Add special use constants for IPv6.
Support IPv4 mapped IPv6 addresses in IPv4 Range.

When an IPv4 scoped range is checked for the presence of an IPv6 address
and the IPv6 address falls within the IPv4 mapped address block
(::ffff:0:0/96) we will automatically convert the IPv6 address to its IPv4
equivalent before making the comparison. This can be helpful on hybrid
dual-stack machines.