Fixed call to sendto() in dhcp6.c. How did it every work before?

configuration - we have to read all the MAC addresses from netlink,
not bail when we find a suitable one.

Fix thinko in dhcp_update_configs - thanks to Hartmut for spotting
that.

Get a sensible address for the default DNS server even when using a
relay.

generalising the DHCP option filter code:
don't match options against context tag  when
context->netid.net == NULL, since there's no tag then.

Build DHCPv6 by default.

First cut at man page changes for DHCPv6