Fix postinst; update fallback DNS servers

- Do not restart dnsmasq in the postinst, because it might have
  unpredictable results.  If dnsmasq is being installed for the
  first time, it will start itself in its postinst.  Otherwise,
  they obviously have a working config.
- Don't use an absolute path for resolvconf in the postinst,
  because Lintian.
- Use Google Public DNS for the fallback DNS servers