networkd: unfoobar serialization of links

networkd: unfoobar serialization of links

We'd start writing an entry line, then another one, then another one,
and then output the rest of the first one, and then some other random
stuff, and the rest of some other lines... Results were ...eh... random.

Let's define a helper to avoid some of the copy&paste madness, and separate
blocks that output a single line with /**********************************/.

This rework doesn't change what data is written, it only tries to fix the
format of the output. The fact that some entries only write data from
link->network, and some from either link->network or link, some stuff only
for dhpc4 leases while some for both dhpc4 and dhcp6, etc, looks rather
suspicious too, but I didn't touch this.