Pass dnsmasq the socket mark to use for listen sockets.

This allows us to configure dnsmasq to reply to DHCP requests and
DNS queries when a VPN is up.

Bug: 37778642
Test: bullhead builds and boots
Test: succesfully tethered when a VPN was connected
Change-Id: I7d5899f80fae856a52a2019550b155bccee2888a