plugin: start a system bus by default

Finit speaks D-Bus itself now and claims org.finit on the system bus
when it finds one, but nothing in a default build ever brings that bus
up.  The plugin that does was opt-in, so the built-in support sat idle
unless the integrator knew to ask for both halves.

Defaulting it on is only reasonable if the result stays the admin's to
change, and a service registered from C through conf_save_service() is
not: it lands in the run path where it cannot be overridden or emptied
out.  So the daemon moves to 20-dbus.conf and its directories to
tmpfiles.d/dbus.conf, the same way hotplug and every other daemon we
ship them for.  The plugin keeps only what has to look at the running
system, the stale pidfile and the machine UUID.

Those directories are no longer chowned to messagebus.  tmpfiles.d
skips a line whose user does not exist rather than falling back, so
the plugin's messagebus/dbus/root ladder has no equivalent there, and
dbus-daemon binds its socket before dropping privileges anyway.

The plugin already bows out where there is no dbus-daemon installed,
so systems that never wanted a bus are unaffected, and
--disable-dbus-plugin is there for those that have one and would still
rather init left it alone.

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

doc/plugins.md

@@ -35,7 +35,10 @@ For your convenience a set of *optional* plugins are available:
 > the `/etc/fstab` file and in the Linux kernel config.
 
 * *dbus.so*: Setup and start system message bus, D-Bus, at boot.
-  _Optional plugin._
+  Enabled by default, and a no-op on systems without a `dbus-daemon`.
+  The daemon is declared in `20-dbus.conf` and its directories in
+  `tmpfiles.d/dbus.conf`, both overridable from `/etc`; the plugin
+  itself only handles what has to inspect the running system.
 
 * *hook-scripts.so*: Trigger the execution of scripts from plugin hook
   points (see [Hooks](#hooks)).  _Optional plugin._