libink/finit: properties, and org.finit on the system bus

Runlevel and version are state, not actions, so they belong behind
org.freedesktop.DBus.Properties rather than another method each.

Finit also claims org.finit on the system bus when it finds one, so
ordinary D-Bus clients can reach it without knowing about
/run/finit/bus.  Opportunistic on purpose: no dbus-daemon is a normal
state for the systems Finit runs on, not an error to report.

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

mkdocs.yml

@@ -56,6 +56,7 @@ nav:
     - Plugins: plugins.md
     - Watchdog: watchdog.md
     - keventd: keventd.md
+    - D-Bus Integration: dbus.md
     - Service State Machine: state-machine.md
     - Distributions: distro.md
     - Requirements: requirements.md