conf: read /run/finit/system before /lib/finit/system

Stanzas in /lib/finit/system cannot use if:keventd, or any other
built-in service: if: is evaluated at parse time, and the generated
.conf files for built-ins were read after the system directory, so the
referenced service did not exist yet and the stanza was dropped.

The late position was deliberate: it kept bundled services from starting
before udev (5d703fd3).  The motivating case was dbus, which has since
moved to a build-time 20-dbus.conf (2e0b1d6f), and the built-ins that
remain want the early slot: watchdogd should start as soon as possible,
keventd is the device manager, and runparts is gated on int/bootstrap so
its position never mattered.

Reading /run/finit/system first also puts the override chain in its
natural order: built-in/generated defaults, then bundled (read-only)
system files in /lib, then the administrator (override) .conf file in
/etc/finit.d, always read last.

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

doc/ChangeLog.md

@@ -8,6 +8,12 @@ All relevant changes are documented in this file.
 
 ### Changes
 
+- The generated `/run/finit/system/*.conf` files for built-in services
+  (keventd, watchdogd, runparts) are now read before
+  `/lib/finit/system/*.conf` instead of after.  Built-ins can now be
+  referenced in `if:` statements from system .conf files and overridden
+  by name from both `/lib/finit/system` and `/etc/finit.d`, the latter
+  is still always read last
 - The `dbus.so` plugin, which starts an external `dbus-daemon`, is now
   enabled by default.  It does nothing on systems without a
   `dbus-daemon` installed, and `--disable-dbus-plugin` opts out.  The