networkd: add support for peer address

This patch adds peer address support for
networkd . In the  [Address]  a new configurable
param is Peer.

[Match]
Name=ipip-tun

[Address]
Address=10.0.0.1/32
Peer=10.0.0.2/32
networkd: netdev - add missing refs

Without this, the underlying device would get freed (and hence
fail).
man: network - document Peer key