dns: forwarding: T3804: bugfix DHCP name-servers used for recursion
dns: forwarding: T3804: bugfix DHCP name-servers used for recursion

Commit 2ecf7a9f9c ('name-server: T3804: merge "system name-servers-dhcp" into
"system name-server"') missed out an old dictionary key "system_name_server_dhcp"
and thus system nameservers configured via DHCP did not get used for the DNS
forwar recursor.

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