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>

libink/dispatch.c

@@ -261,6 +261,7 @@ const char *link_call_path     (const link_call_t *c) { return c ? c->incoming.p
 const char *link_call_interface(const link_call_t *c) { return c ? c->incoming.interface : NULL; }
 const char *link_call_member   (const link_call_t *c) { return c ? c->incoming.member    : NULL; }
 uid_t       link_call_uid      (const link_call_t *c) { return c ? c->uid               : LINK_UID_UNKNOWN; }
+link_connection_t *link_call_connection(const link_call_t *c) { return c ? c->conn : NULL; }
 
 link_writer_t *link_call_reply(link_call_t *call)
 {