DNS: add database key slirp/resolver
DNS: add database key slirp/resolver

If slirp/resolver = host then we will always use the host resolver,
otherwise we will use the upstream servers named in slirp/dns.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit