dns: Better propagate known DNS servers

dns_servers_update() is a new function that must be called
when ever there are changes to the DNS server configuration.
This may be by the user or by learning new DNS servers from
a dial-up connection, etc.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>