base-files: Fix race-conditions with IPv6 sysctls

git-svn-id: svn://svn.openwrt.org/openwrt@35368 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: remove IPv6 forwarding-sysctl workaround

git-svn-id: svn://svn.openwrt.org/openwrt@35369 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: remove IPv6 forwarding-sysctl workaround

git-svn-id: svn://svn.openwrt.org/openwrt@35370 3c298f89-4303-0410-b956-a3cf2f4a3e73
6relayd: remove IPv6 forwarding-sysctl workaround

git-svn-id: svn://svn.openwrt.org/openwrt@35371 3c298f89-4303-0410-b956-a3cf2f4a3e73