Fix for bug 10743

Fix for bug 10743

This adds in the option to have "deny known clients" in dhcpd.conf
This is applied to the range command so applies to the dynamic addresses
given.
If you have just a range statement say in blue then if you are not using
vlans you could have the situation where a known host in green might end
up getting a lease from the blue range. Here a deny known-clients makes
sense. Your range in this case would be limited to only unknown clients if
deny known-clients was selected.
dhcp WUI has been modified to add in this command. Error message has been
added to check that a range has been specified if the deny unknown clients
checkbox has been selected.
Language files updated with additional items (English, German & Dutch).

For more information on the history of this please see the bugzilla entry
Signed-off-by: Adolf Belka <ahb.ipfire@gmail.com>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>