Send TCP DNS messages in one write() call. Stops TCP stream fragmenting.

This is an optimisation, not a bugfix. Thanks to Jim Bos for spotting it.
Fix crash on SERVFAIL when --conntrack in use.
Fix regression in dhcp_lease_time utility.
Manpage typos.
Note that dhcp_lease_time and dhcp_release only work for IPv4.
--dhcp-match et al now work with BOOTP as well as DHCP.
Add --synth-domain
Allow option prefix in --synth-domain.
Check length of synth-domain prefix.
Fix FTBFS in ipset.c with old kernel headers.