dns-forwarding.py: be more tolerant on missconfigured DHCP source
dns-forwarding.py: be more tolerant on missconfigured DHCP source

If a user configures 'set service dns forwarding dhcp <if>' and <if> is not a
DHCP interface, this won't corrupt the recursor.conf.

The missconfiguration is simply ignored.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x