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>

src/service.h

@@ -59,6 +59,7 @@ void      service_forked         (svc_t *svc);
 void      service_ready          (svc_t *svc, int ready);
 
 int       service_stop           (svc_t *svc);
+int       service_reload         (svc_t *svc);
 int       service_step           (svc_t *svc);
 void      service_step_all       (int types);
 void      service_worker         (void *unused);