base-files: network.sh: extend network_get_ipaddr6() / network_get_subnet6() to take ipv6-prefix-assignment into account, add further helper functions to fetch all ips of a given interface

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36942 3c298f89-4303-0410-b956-a3cf2f4a3e73