Add support for custom dnsmasq config and odhcp6c handler scripts

package/gluon-core/files/etc/odhcp6c.user

+for file in /lib/gluon/odhcp6c.d/*; do
+	[ ! -x $file ] || $file
+done

package/gluon-core/files/lib/gluon/upgrade/core/invariant/010-dnsmasq.d

+#!/bin/sh
+
+echo 'conf-dir=/lib/gluon/dnsmasq.d' > /etc/dnsmasq.conf