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>

man/initctl.8

@@ -19,6 +19,10 @@ is the official tool for interacting with
 .Xr finit 8 .
 It comes with functions for showing status, querying health of services,
 as well as editing, enabling and disabling services.
+Commands are sent over the Finit D-Bus API when available, with the
+legacy
+.Pa INIT_SOCKET
+protocol as fallback.
 .Sh OPTIONS
 This program follows the usual UNIX command line syntax, with long
 options starting with two dashes (`-').  The options are as follows:
@@ -205,6 +209,10 @@ restricted
 Send signal S to service, by name.
 .Cm S
 may be a complete signal name such as SIGHUP, or short name such as HUP, or the signal number such as 1 (SIGHUP)
+.It Nm Ar monitor
+Stream service state, runlevel, and condition change signals from the
+Finit D-Bus API until interrupted.  Only available when Finit is built
+with D-Bus support
 .It Nm Ar restart Cm NAME[:ID]
 Restart (stop/start) service by name
 .It Nm Ar status Cm NAME[:ID]