shared: add minimal varlink implementation

This adds a minimal Varlink (https://varlink.org/) implementation to our
tree. Given that we already have a JSON logic it's an easy thing to add.

Why bother?

We currently have major problems with IPC before dbus-daemon is up, and
in all components that dbus-daemon itself makes use of (such as various
NSS modules to resolve users as well as the journal which dbus-daemon
logs to). Because of that we so far ended up creating various (usually
crappy) work-arounds either coming up with secondary IPC systems or
sharing data statelessly in /run or similar. Let's clean this up, and
instead use a clean, well-defined, broker-less IPC for cases like that.

This is a minimal implementation of Varlink, i.e. the most basic logic
only. Stuff that's missing is left out on purpose: there's no
introspection/validation and there's no name service. It might make
sense to add that later, but for now let's only do the minimum buy-in we
can get away with. In particular as I'd assume that at least initially
we only use this IPC for our internal communication avoiding
introspection and the name service should be fine.

Specifically, I'd expect that we add IPC interfaces to the following
concepts with this scheme:

1. nss-resolve (so that hostname lookups with resolved work before
   resolved is up)
2. journald (so that IPC calls to journald don't have to go through
   dbus-daemon thus creating a cyclic dependency between journald and
   dbus-daemon)
3. nss-systemd (so that dynamic user lookups via PID 1 work sanely even
   inside of dbus-daemon, because otherwise we'd want to use dbus to run
   dbus which causes deadlocks)
4. networkd (to make sure one can talk to it in the initrd already,
   long before dbus is around)

And there might be other cases similar to this.

src/shared/meson.build

@@ -164,6 +164,8 @@ shared_sources = files('''
         uid-range.c
         uid-range.h
         utmp-wtmp.h
+        varlink.c
+        varlink.h
         verbs.c
         verbs.h
         vlan-util.c