client - properly update the dns resolver for debian and ubuntu

add these to the openvpn config:

script-security 2
up /etc/openvpn/update-resolv-conf
down /etc/openvpn/update-resolv-conf

see https://we.riseup.net/leap+client/openvpn-dns for a longer discussion of openvpn, debian/ubuntu, and dns

(from redmine: created on 2013-01-03, closed on 2013-01-22, relates #1470 (closed), relates #1454, precedes #1471 (closed))