File group and mode for device nodes.

Enable optional configuration of file group and mode for device nodes. The
options are --with-dev-group=GROUP and --with-udev-mode=MODE.
diff --git a/Makefile.am b/Makefile.am
index 9e2762d..c6d46c2 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -16,7 +16,7 @@
 libmtp.usermap: util/mtp-hotplug
 	util/mtp-hotplug > libmtp.usermap
 @UDEV_RULES@: util/mtp-hotplug
-	util/mtp-hotplug -u -p"@UDEV@" > @UDEV_RULES@
+	util/mtp-hotplug -u -p"@UDEV@" @UDEV_GROUP@ @UDEV_MODE@ > @UDEV_RULES@
 libmtp.fdi: util/mtp-hotplug
 	util/mtp-hotplug -H > libmtp.fdi