The userns package in libcontainer was integrated into the moby/sys/user
module at commit [3778ae603c706494fd1e2c2faf83b406e38d687d][1].
This patch deprecates the containerd fork of that package, and adds it as
an alias for the moby/sys/user/userns package.
[1]: 3778ae603c
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
