[patch v3] Allow for unpriviledged nested containers

[Florian Weimer]

* DJ Delorie:

> Subject: Allow for unpriviledged nested containers
>
> When running a "make check" in an untrusted podman container, we do
> not have priviledges to mount a new /proc.  Previously, we just failed
> to initialize the container and thus all test-container tests were
> "unsupported".  With this change, we bind mount the parent's /proc,
> which we have priviledges to do.  Note that MS_REC is needed as /proc
> typically has things mounted within it, and not mounting those would
> be a security hole[*].

I see a new test failure with this, elf/tst-pldd.  Happens with
kernel-5.14.17-301.fc35.x86_64, kernel-5.14.13-100.fc33.x86_64,
linux-image-5.10.0-9-amd64_5.10.70-1, as a non-root user.

Thanks,
Florian