keventd: serve org.finit.Device1 on /run/keventd/bus

The udev parity gaps that were blocked on IPC -- settle, trigger,
info, queue introspection, runtime rule reload -- become bus methods.
keventd serves its own socket the way Finit serves /run/finit/bus:
brokerless, libink, one socket per daemon, no forwarding between the
two.

  Settle(u) -> b      parked until the queue drains or the timeout
                      passes; true when settled
  Trigger(s, s)       replay events, action + subsystem glob
  Info(s) -> a{ss}    /run/udev/data properties for a devpath
  RulesReload() -> u  re-read rules dirs, returns rule count
  QueueEmpty (b), SeqnumProcessed (t) properties
  DeviceProcessed (ss) signal after each fully handled event

The queue state is the highest kernel seqnum keventd has handled,
baselined at startup, against /sys/kernel/uevent_seqnum.  keventd -S
now asks the running daemon first and falls back to seqnum polling.
In passive mode Trigger and RulesReload refuse.  Adds
link_call_connection() for the park bookkeeping.

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

keventd/keventd.h

@@ -163,6 +163,22 @@ int             coldplug         (void);
 int             coldplug_trigger (const char *action, const char *subsys_glob);
 int             uevent_action_valid(const char *action);
 
+/* keventd.c: queue and rules state, for the D-Bus layer */
+unsigned long long kev_seq_processed(void);
+void            kev_seq_baseline (void);
+int             kev_queue_empty  (void);
+int             kev_rules_reload (void);
+
+#ifdef HAVE_DBUS
+#include <uev/uev.h>
+
+/* dbus.c: org.finit.Device1 on /run/keventd/bus */
+int             devbus_init      (uev_ctx_t *ctx, int passive);
+void            devbus_exit      (void);
+void            devbus_notify    (const char *devpath, const char *action);
+int             devbus_client_settle(int timeout_s);
+#endif
+
 #endif /* FINIT_KEVENTD_H_ */
 
 /**