vyatta-cfg-system: expand 'set system allow-dhcp-nameservers' logic

Expand the logic in 'set system allow-dhcp-nameservers', so that in
addition to either allowing / denying DHCP related updates to
resolv.conf when the interface changes state, it will also either
add or remove the currently supplied DHCP name-servers depending on
it's state.  This change in name-servers will take place immediately
on commit.

Bug #309 http://bugzilla.vyos.net/show_bug.cgi?id=309