This migrates uses of github.com/opencontainers/runc/libcontainer/user
to the new github.com/moby/sys/user module, which was extracted from
runc at commit [opencontainers/runc@a3a0ec48c4].
This is the initial release of the module, which is a straight copy, but
some changes may be made in the next release (such as fixing camel-casing
in some fields and functions (Uid -> UID).
[opencontainers/runc@a3a0ec48c4]: a3a0ec48c4
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
