Migrate `syscall` --> `golang.org/x/sys/unix`, redux
Migrate `syscall` --> `golang.org/x/sys/unix`, redux

This completes the migration from `syscall` --> `golang.org/x/sys/unix`
which was started in https://github.com/vishvananda/netns/pull/40:
> The syscall package is deprecated and no longer updated as per
https://golang.org/pkg/syscall/. Use the golang.org/x/sys/unix package
instead.
vishvananda/netns