Improve the preseed files' handling of /etc/network/interfaces by having them set the "auto" bringup option for a given interface if and only if the interface can be brought up via dhclient during deployment. If an interface exists but cannot be brought up in this way, its entry is created, but the "auto" option is commented out and a comment advising that cabling be checked is added. This should improve deployment reliability when only some NICs are cabled.