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>

libink/path.h

@@ -0,0 +1,21 @@
+/* libink — D-Bus object-path encoding for arbitrary identifiers.
+ *
+ * Copyright (c) 2026  Joachim Wiberg <troglobit@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+#ifndef LIBINK_PATH_H_
+#define LIBINK_PATH_H_
+
+#include <stddef.h>
+
+/* Encode `in` as a D-Bus path segment.  Bytes in [A-Za-z0-9] pass
+ * through unchanged; everything else (including '_' itself) becomes
+ * "_HH" with lowercase hex, mirroring systemd's escape_path.
+ *
+ * Returns the encoded length (excluding the terminating nul), or -1
+ * if the output buffer cannot fit the result.  `outsz` must
+ * accommodate the encoded bytes plus a trailing nul; the worst-case
+ * size for an N-byte input is 3*N + 1. */
+int link_path_encode(const char *in, char *out, size_t outsz);
+
+#endif /* LIBINK_PATH_H_ */