[packages] strongswan: introduce /etc/ipsec.user file

enable user to add their own ip (or other) rules using
/etc/ipsec.user file on events like IPsec tunnel state change

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@36462 3c298f89-4303-0410-b956-a3cf2f4a3e73