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/log.c

@@ -0,0 +1,40 @@
+/* libink — optional debug tracing, routed to whoever embeds us.
+ *
+ * libink has no logger of its own by design: it must not depend on the
+ * host's logging, and PID 1 already has one.  The embedder installs a
+ * callback and gets a line per connection, call, and authorization
+ * decision.  Formatting happens before the callback, so an embedder
+ * that wants tracing off should uninstall rather than filter.
+ *
+ * Copyright (c) 2026  Joachim Wiberg <troglobit@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+
+#include "internal.h"
+
+static link_log_cb_t  logger;
+static void          *logger_userdata;
+
+void link_set_logger(link_log_cb_t cb, void *userdata)
+{
+	logger          = cb;
+	logger_userdata = userdata;
+}
+
+void __log(const char *func, const char *fmt, ...)
+{
+	char    msg[256];
+	va_list ap;
+
+	if (!logger)
+		return;
+
+	va_start(ap, fmt);
+	vsnprintf(msg, sizeof(msg), fmt, ap);
+	va_end(ap);
+
+	logger(logger_userdata, func, msg);
+}