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>

doc/dbus.md

@@ -22,6 +22,10 @@ Bus address
 | Local (always)         | `unix:path=/run/finit/bus`                  |
 | System (opportunistic) | `unix:path=/var/run/dbus/system_bus_socket` |
 
+The bundled device manager serves its own `org.finit.Device1` the same
+way on `unix:path=/run/keventd/bus`, see [keventd](keventd.md); one
+socket per daemon, no forwarding between them.
+
 The **local** bus is brokerless: clients connect straight to Finit over a
 Unix-domain socket using the standard D-Bus SASL EXTERNAL handshake.  No
 `dbus-daemon` is required, which makes it suitable for embedded systems that
@@ -133,6 +137,7 @@ All read-only strings; observable via `Properties.Get` and
 |-----------------------|----------------------------------------|------------------------|
 | `ServiceStateChanged` | `sss` — identity, old state, new state | Service transitions    |
 | `RunlevelChanged`     | `ss`  — old level, new level           | System runlevel change |
+| `ConfigReloaded`      | —                                      | `initctl reload` completed; external condition providers re-assert |
 
 State names emitted by `ServiceStateChanged` are stable wire strings:
 `halted`, `done`, `dead`, `cleanup`, `teardown`, `stopping`, `setup`,