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/build.md

@@ -53,14 +53,16 @@ Below are a few of the main switches to configure:
 
 * `--disable-dbus`: Opt out of Finit's built-in D-Bus support, enabled by
   default.  See [D-Bus Integration](dbus.md) for what it provides.  Not to
-  be confused with `--enable-dbus-plugin` below, which only starts an
-  external `dbus-daemon`
+  be confused with `--disable-dbus-plugin` below, which only governs
+  starting an external `dbus-daemon`
 
 * `--enable-alsa-utils-plugin`: Enable the optional `alsa-utils.so` sound plugin.
 
-* `--enable-dbus-plugin`: Enable the optional D-Bus `dbus.so` plugin, which
-  launches `dbus-daemon` at boot.  Unrelated to the built-in bus, see
-  `--disable-dbus` above.
+* `--disable-dbus-plugin`: Drop the `dbus.so` plugin, which launches
+  `dbus-daemon` at boot.  Enabled by default; the plugin does nothing on
+  a system that has no `dbus-daemon` installed, so opting out is only
+  needed to keep init from starting a bus on a system that has one.
+  Unrelated to the built-in bus, see `--disable-dbus` above.
 
 * `--enable-resolvconf-plugin`: Enable the `resolvconf.so` optional plugin.