luci-app-ddns: modified to support ddns-scripts 2.1.0-2

luci-app-ddns: modified to support ddns-scripts 2.1.0-2

- moved /usr/lib/ddns/dynamic_dns_lucihelper.sh to ddns-scripts package
- fixed error message when validating proxy
- modified validating ip_script to allow the usage of parameters
- using ipkg/postinst AND ipkg/postinst-pkg as long as LuCI Makefiles did not
	fullfil requirements of new OpenWrt default_postinst and default_postrm
	reported in Issue #255
- cleanup whitespaces at line ends

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>