libink: trace connections, calls, and authorization decisions

The legacy socket logs a line per command under initctl debug; the bus
logged nothing, so the transport that now carries most of initctl was
the one you could not watch.

libink gets a logger hook rather than a dependency on Finit's: it
passes the emitting function and a formatted message, and dbus.c hands
both to logit() so the two sources read alike.  Trace points cover the
connection lifecycle, every inbound call, and why a call was refused.

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

libink/internal.h

@@ -160,6 +160,11 @@ struct link_connection {
 	struct link_server  *server;	/* back-pointer for dispatch */
 };
 
+/* log.c — tracing, no-op unless the embedder installed a callback. */
+void __log(const char *func, const char *fmt, ...)
+	__attribute__((format(printf, 2, 3)));
+#define __dbg(fmt, ...)  __log(__func__, fmt, ##__VA_ARGS__)
+
 /* io.c — shared EINTR-resilient I/O loops. */
 int  __io_write_all(int fd, const void *buf, size_t len);
 int  __io_read_full(int fd, void *buf,       size_t len);