The new switch operates like --network-veth, but may be specified
multiple times (to define multiple link pairs) and allows flexible
definition of the interface names.
This is an independent reimplementation of #1678, but defines different
semantics, keeping the behaviour completely independent of
--network-veth. It also comes will full hook-up for .nspawn files, and
the matching documentation.
.nspawn fiels are simple settings files that may accompany container
images and directories and contain settings otherwise passed on the
nspawn command line. This provides an efficient way to attach execution
data directly to containers.