bug#31977: clone tests fail on CentOS 7

[Paul Garlick <pgarlick@tourbillion-technology.com>]

Hi Simon,

> If I understand well your message:
> 
>         Files:
>         a) '/proc/self/ns/user' exists
>         b) '/proc/sys/kernel/unprivileged_userns_clone' does not exist.

Yes, this is the case on CentOS.  

So testing for the existence of the unprivileged_userns_clone file is
an insufficent test for unprivileged user namespaces.  We have learnt
that trying to create the file as a dummy file does not work, since the
/proc/sys/kernel directory is read-only even for root.

So the current 'unprivileged-user-namespace-supported?' function in
gnu/build/linux-container.scm really only works for Debian-derived
systems.  Other systems, that co not create the
unprivileged_userns_clone file, differ in their default configurations.
CentOS, for example, disables the feature.  However, Guix System
enables it.

It has been suggested that the feature itself should be tested, instead
of relying on the /proc filesystem.  This could well be a better idea
and I gather from the thread that this idea is being worked on.  I can
test on CentOS when a new patch is available.

Best regards,

Paul.