libink: a brokerless D-Bus implementation for Finit

Finit had no way to answer the question every service manager gets
asked: what is running, and change it.  D-Bus is how the rest of
userspace asks, but linking libdbus, sd-bus or GIO into PID 1 buys a
dependency, an allocator and a main loop we do not control.

So libink: the wire format, an object tree, and a bus of Finit's own
at /run/finit/bus, gated like INIT_SOCKET.  It speaks the standard
org.freedesktop.DBus, .Peer, .Introspectable interfaces, and Finit's
own Manager1, Service1 and Cond1 on top.  Methods that change
something are marked privileged and answered only for a caller the
kernel vouched for, via SO_PEERCRED.

Server and client both, since initctl is the first thing that needs
to talk to it, and its Start/Stop/Restart/Reload now go over the bus
rather than the legacy socket.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Makefile.am

@@ -1,5 +1,16 @@
 ACLOCAL_AMFLAGS    = -I m4
-SUBDIRS            = man plugins src system tmpfiles.d
+
+# libink must precede src in SUBDIRS because finit links against
+# libink at build time.  Automake recurses subdirs strictly in
+# declaration order, so the typical "explicit dependency" pattern
+# (foo: bar) doesn't help here — ordering is the only thing that
+# does.  libsystemd is consumed by test/serv only, so its position
+# after src is fine.
+SUBDIRS            = man plugins
+if DBUS
+SUBDIRS           += libink
+endif
+SUBDIRS           += src system tmpfiles.d
 dist_doc_DATA      = README.md LICENSE contrib/finit.conf
 
 if CONTRIB