ipset: Integrate ipset.c into build system.

Old kernels use HAVE_OLD_IPSET while newer kernels use HAVE_IPSET.
ipset: Parse new --ipset option and match domains in forward.c

The matching logic is the same as for --address. extract_address is
responsible for calling out to add_to_ipset.
ipset: Update man page and example config to reflect new option.