config: improved allowed update mask/defaults

DHCPv6 protocol does not define routing options (-> RA),
current linux ypbind does not support ipv6, ... do not
confuse users showing the current config would do this.
Revert "Merge pull request #441 from mtomaschewski/allow-update-mask"
Merge pull request #441 from mtomaschewski/allow-update-mask