base-files: Set default value for IPv6 forwarding

git-svn-id: svn://svn.openwrt.org/openwrt@35299 3c298f89-4303-0410-b956-a3cf2f4a3e73
base-files: fix network_get_prefix6

git-svn-id: svn://svn.openwrt.org/openwrt@35300 3c298f89-4303-0410-b956-a3cf2f4a3e73