dhclient.conf should set send DHCPv6 host-name option

[TJ]

Despite the fix in 2012 (bug #991360) the default configuration shipped with isb-dhcp-client /etc/dhcp/dhclient.conf in 2021 still does not set the DHCPv6 hostname option by default. All it needs is:

# send host-name with DHCPv6 not just DHCPv4
send fqdn.fqdn = gethostname();

Without this option it causes dhcpy6d's dynamic DNS update (to bind9) to fail because the hostname is empty.