Bug: dhcpd needs ddns-hostname to update dynamic hostnames via ddns

[dschier-wtd]

Du to whatever behaviour, ddns does not work, when the host has gotten a dynamic hostname.
If the host does not have a static hostname ddns is not updated. One has to add the option "ddns-hostname" and optionally "ddns-domain" to the host declaration to force the ddns update, even when the host is getting it's hostname via dhcp.