It's time. sd-json was already done earlier in this cycle, let's now
make sd-varlink public too.
This is mostly just a search/replace job of epical proportions.
I left some functions internal (mostly IDL handling), and I turned some
static inline calls into regular calls.
Let's get networkd onto Varlink. This only adds the most basic of
operations.
I'd love to see networkd do Varlink for all its basic operations so that
networkctl can use that, and work correctly before D-Bus is up. Right
now, many of networkctls calls simply don't work before D-Bus, and I'd
like to see that improved.