This was initially added by commit d78ee47 but later
moved from libcontainer/configs to libcontainer/devices by
commit 677baf2.

Looks like since commit 677baf2 and also [1]
there is no use for this, thus removing.

[1] containers/buildah#2652

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Commit bf74951 added these two functions, but they are only used from
Windows code. The v1 of this patch moved these functions to _windows.go
file, but after some discussion we decided to drop windows code
altogether, so this is what this patch now does.

This fixes

> libcontainer/user/user.go:64:6: func `groupFromOS` is unused (unused)
> libcontainer/user/user.go:35:6: func `userFromOS` is unused (unused)

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Move the unix-specific code to a file that's not compiled on
Windows.

Some of the errors (ErrUnsupported, ErrNoPasswdEntries, ErrNoGroupEntries)
are used in other parts of the code, so are moved to a non-platform
specific file.

Most of "user" is probably not useful on Windows, although it's possible
that Windows code may have to parse a passwd file, so leaving that code
for now.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>