If importing an archive via stdin write it to a temporary file such that
the temporary file can be opened multiple times later on. Otherwise, we
may end up with an empty image.
Also fix a bug in the URL parsing code; we need to check whether there's
actually a scheme.
Add system tests for `podman import` exercising the basics.
Fixes: #10994
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>
