Add hash:net,net ipset for storage of v4/v6 CIDR pairs.

This adds a new set that provides the ability to configure pairs of
subnets.

In order to achieve this, additional handling code has been
added to deal with the fact that both parameters accept an arbitrary
CIDR range. A preprocessor symbol is used to conditionally enable this
extra code.

Signed-off-by: Oliver Smith <oliver@8.c.9.b.0.7.4.0.1.0.0.2.ip6.arpa>