com.docker.slirp: don't hardcode 10.0.75.1 for upstream DNS
com.docker.slirp: don't hardcode 10.0.75.1 for upstream DNS

On Windows we currently supply an argument `--dns 10.0.75.1`
which points at another local DNS forwarder. There's no need to
hard-code the policy here. It's also the wrong policy for Mac.

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit