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>

test/check.sh

@@ -25,8 +25,8 @@ if [ "$run_make" -eq 1 ]; then
 fi
 
 ./configure --prefix=/usr --exec-prefix= --sysconfdir=/etc --localstatedir=/var	\
-	    --enable-x11-common-plugin --enable-testserv-plugin --with-watchdog \
-            --with-keventd --with-libsystemd                                    \
+	    --enable-dbus --enable-x11-common-plugin --enable-testserv-plugin   \
+	    --with-watchdog --with-keventd --with-libsystemd                    \
 	    CFLAGS='-fsanitize=address -ggdb'
 
 if [ "$run_make" -eq 1 ]; then