systemd-link: add support to configure the device port (#6153)

      This work allows to configure device port:

tp — An Ethernet interface using Twisted-Pair cable as the medium.
aui — Attachment Unit Interface (AUI). Normally used with hubs.
bnc — An Ethernet interface using BNC connectors and co-axial cable.
mii — An Ethernet interface using a Media Independent Interface (MII).
fibre — An Ethernet interface using Optical Fibre as the medium.