dbus: install the bus policy in $datadir/dbus-1/system.d

make distcheck fails in install:

    /usr/bin/install: cannot create regular file '/etc/dbus-1/system.d/org.finit.conf': Permission denied

The policy was installed to $sysconfdir/dbus-1/system.d.  distcheck only
overrides the prefix, so the file escaped its sandbox and aimed for the
real /etc.

Install it where dbus looks for package owned policy, leaving
/etc/dbus-1/system.d to the admin.  The test bus config reads it
relative to itself.

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

doc/dbus.md

@@ -43,8 +43,9 @@ connections are refused until one disconnects.  Match rules given to
 `arg0` and its variants are not implemented.
 
 For the system bus a standard busconfig policy ships in
-`dbus-1/org.finit.conf`: only root may own `org.finit`, unprivileged
-callers reach introspection, properties, and the read-only methods.
+`dbus-1/org.finit.conf`, installed to `$datadir/dbus-1/system.d`: only
+root may own `org.finit`, unprivileged callers reach introspection,
+properties, and the read-only methods.
 Finit also enforces per-method authorization itself, so a permissive
 policy installed by mistake does not open state-changing methods.