Allow OverlayFS whiteouts to be converted to AUFS, by filtering a tar
stream.

In this direction the conversion is a single pass, single operation.
There is no requirement to pre-scan the tar.

Close sylabs#53

Prior to this PR, when a tar file contains an entry prefixed with `./`
that is processed by a whiteout transformation, the `./` prefix is lost.

This loss of the prefix was due to the use of `filepath` functions that
clean the resulting path. Instead, use `filepath.Split` and manual
concatenation with filepath.Seperator` to avoid the clean.