copier: work around freebsd bug for "mkdir /"
copier: work around freebsd bug for "mkdir /"

On FreeBSD, os.Mkdir can return EISDIR if the target is the root. As a
workaround, treat EISDIR as a non-error similarly to EEXISTS.

[NO NEW TESTS NEEDED]

Signed-off-by: Doug Rabson <56be4c1e9e32abdc1e263227d75b7c1114499099@rabson.org>

nalind/buildah-1,TomSweeneyRedHat/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah-1,TomSweeneyRedHat/buildah