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/initctl.md

@@ -5,6 +5,11 @@ Finit also implements a modern API to query status, and start/stop
 services, called `initctl`.  Unlike `telinit` the `initctl` tool does
 not return until the given command has fully completed.
 
+Since Finit v5, `initctl` talks to Finit over the built-in
+[D-Bus API](dbus.md) when the bus socket is available, falling back
+to the legacy `INIT_SOCKET` transport.  The `monitor` command streams
+the `org.finit` signals until interrupted.
+
 ```
 Usage: initctl [OPTIONS] [COMMAND]
 
@@ -14,6 +19,7 @@ Options:
   -f, --force               Ignore missing files and arguments, never prompt
   -h, --help                This help text
   -j, --json                JSON output in 'status' and 'cond' commands
+  -n, --noerr               Ignore error, e.g., already started/enabled/...
   -1, --once                Only one lap in commands like 'top'
   -p, --plain               Use plain table headings, no ctrl chars
   -q, --quiet               Silent, only return status of command
@@ -26,15 +32,8 @@ Commands:
   help                      This help text
   version                   Show program version
 
-  ls | list                 List all .conf in /etc/finit.d
-  create   <CONF>           Create   .conf in /etc/finit.d/available
-  delete   <CONF>           Delete   .conf in /etc/finit.d/available
-  show     <CONF>           Show     .conf in /etc/finit.d/available
-  edit     <CONF>           Edit     .conf in /etc/finit.d/available
-  touch    <CONF>           Change   .conf in /etc/finit.d/available
-  enable   <CONF>           Enable   .conf in /etc/finit.d/available
-  disable  <CONF>           Disable  .conf in /etc/finit.d/enabled
-  reload                    Reload  *.conf in /etc/finit.d (activate changes)
+  show                      Show     /etc/finit.conf
+  reload                    Reload   /etc/finit.conf (activate changes)
 
   cond     set   <COND>     Set (assert) user-defined conditions     +usr/COND
   cond     get   <COND>     Get status of user-defined condition, see $? and -v
@@ -50,6 +49,7 @@ Commands:
                             Note: Finit .conf file(s) are *not* reloaded!
   restart  <NAME>[:ID]      Restart (stop/start) service by name
   kill     <NAME>[:ID] <S>  Send signal S to service by name, with optional ID
+  monitor                   Stream D-Bus signals (service state, conditions) until ^C
   ident    [NAME]           Show matching identities for NAME, or all
   status   <NAME>[:ID]      Show service status, by name
   status                    Show status of services, default command
@@ -65,9 +65,10 @@ Commands:
   halt                      Halt system
   poweroff                  Halt and power off system
   suspend                   Suspend system
-  switch-root NEWROOT [INIT]  Switch to new root filesystem (initramfs only)
+  switch-root ROOT [INIT]   Switch to new root filesystem (initramfs)
 
   utmp     show             Raw dump of UTMP/WTMP db
+
 ```
 
 For services *not* supporting `SIGHUP` the `<!>` notation in the .conf