systemd: slurp bugfixes in from master

This pulls in all patches marked with a Backport note as well as a some
assorted networkd fixes. May this release be a little less crashy...

Patches 1-29 are just slurped in because they have a Backport/bugfix note. 30-36 are assorted other networkd fixes. `215-0034-networkd-netdev-add-missing-refs.patch` is the particularly important one, fixing the networkd crashes we've been seeing.