podman cp: support copying on tmpfs mounts

Traditionally, the path resolution for containers has been resolved on
the *host*; relative to the container's mount point or relative to
specified bind mounts or volumes.

While this works nicely for non-running containers, it poses a problem
for running ones.  In that case, certain kinds of mounts (e.g., tmpfs)
will not resolve correctly.  A tmpfs is held in memory and hence cannot
be resolved relatively to the container's mount point.  A copy operation
will succeed but the data will not show up inside the container.

To support these kinds of mounts, we need to join the *running*
container's mount namespace (and PID namespace) when copying.

Note that this change implies moving the copy and stat logic into
`libpod` since we need to keep the container locked to avoid race
conditions.  The immediate benefit is that all logic is now inside
`libpod`; the code isn't scattered anymore.

Further note that Docker does not support copying to tmpfs mounts.

Tests have been extended to cover *both* path resolutions for running
and created containers.  New tests have been added to exercise the
tmpfs-mount case.

For the record: Some tests could be improved by using `start -a` instead
of a start-exec sequence.  Unfortunately, `start -a` is flaky in the CI
which forced me to use the more expensive start-exec option.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

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