If CNI is about to configure an IPv6 address, make sure IPv6 is not
disabled through the "disable_ipv6" sysctl setting.
This is a workaround for Docker 17.06 and later which sets
disable_ipv6=1 for all interfaces even if ipv6 is enabled in Docker.
This change improves the performance of the ipam.ConfigureIface.
Some plugins are slow because of the ip.SettleAddress in ipam.ConfigureIface.
It seems to be only needed for IPv6, so should be skipped if only IPv4 is used.
- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility
