doc: complete and cross-link the D-Bus documentation

dbus.md was orphaned: not in dist_docs_DATA, not linked from the
user guide.  Wire it into the doc dist and link it from the index
features list, features.md, initctl.md, and plugins.md, where the
dbus.so plugin entry now disambiguates the external system bus from
the built-in org.finit API.

Document the 64-peer cap, the supported AddMatch keys, the busconfig
policy file, the legacy-parity edge semantics with the deliberate
SetRunlevel InvalidArgs divergence, the reload-signal behavior of
Service1.Reload, and the reboot family timeout.  Refresh the stale
initctl.md usage paste, add monitor and the D-Bus transport to
initctl(8), add /run/finit/bus to the filesystem layout, and flatten
the ChangeLog D-Bus entry to house style.

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

doc/features.md

@@ -275,6 +275,10 @@ initctl top                       # Interactive resource monitor
 initctl cond set usr/custom       # Set custom condition
 ```
 
+`initctl` rides Finit's built-in [D-Bus API](dbus.md) when available,
+which is also open to any other bus client, including `dbus-send` and
+language bindings.
+
 See the [Commands & Status](initctl.md) section for complete documentation.