refine init script about openwrt-chinadns-ng

[pexcn]

get_value() {
	local section="$1"
	local option="$2"
	local default="$3"
	local _loctmp
	config_get _loctmp "$section" "$option" "$default"
	[ -n "$_loctmp" ] || return 0
	echo "$_loctmp"
}

get_value_bool() {
	local section="$1"
	local option="$2"
	local default="$3"
	local _loctmp
	config_get_bool _loctmp "$section" "$option" "$default"
	[ -n "$_loctmp" ] || return 0
	echo "$_loctmp"
}