dbus: gate the bus socket like INIT_SOCKET

The D-Bus socket was bound world read/write, on the reasoning that
SO_PEERCRED authorizes each method anyway.  That leaves the read-only
surface open to every local user, and it quietly ignores --with-group:
a system that restricts initctl to the wheel group still handed the
same service state to anyone who asked over the bus.

Bind it 0660 and chown it to the configured group, the same gate the
fallback socket has always had.  libink takes the mode as an argument
rather than assuming one, since who may connect is the embedder's
policy, not the library's.

The mode is applied at bind(), so there is no window where the socket
is more permissive than intended.

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

doc/dbus.md

@@ -27,6 +27,12 @@ Unix-domain socket using the standard D-Bus SASL EXTERNAL handshake.  No
 `dbus-daemon` is required, which makes it suitable for embedded systems that
 don't ship one.
 
+The socket is `0660`, owned by `root` and the group given to
+`--with-group` at build time, the same gate as `/run/finit/socket` that
+`initctl` falls back on.  The bus reaches every operation `initctl`
+does, so restricting one and not the other would leave the door open.
+Members of that group may use it, see [Authorization](#authorization).
+
 The **system** bus is best-effort: Finit probes for a running `dbus-daemon`
 and, when reachable, claims the well-known name `org.finit` so that standard
 tooling sees Finit just like any other system service: