Add support for raw IP interface type

Under linux some devices can expose raw IP interfaces, such as WWAN modems. In that case IP data is not encapsulated in any lower level protocol.

dhclient does not support this currently and this patch adds support for such pure IP interfaces.

The original patch comes from Bjørn Mork on Network-Manage mailing list: https://mail.gnome.org/archives/networkmanager-list/2015-December/msg00044.html