Allow smaller than 64 prefix lengths in synth-domain, with caveats.
--synth-domain=1234:4567::/56,example.com is now valid.

Make domains generated by --synth-domain appear in replies
when in authoritative mode.

Ensure CAP_NET_ADMIN capability is available when
conntrack is configured. Thanks to Yick Xie for spotting
the lack of this.

When --dhcp-hostsfile --dhcp-optsfile and --addn-hosts are
given a directory as argument, define the order in which
files within that directory are read (alphabetical order
of filename). Thanks to Ed Wildgoose for the initial patch
and motivation for this.