pxar: encoder: limit number of max entries held at once in memory during archive...

pxar: encoder: limit number of max entries held at once in memory during archive creation.

Limit the total number of entries and therefore the approximate memory
consumption instead of doing this on a per directory basis as it was previously.
This makes more sense as it limits not only the width but also the depth of the
directory tree.

Further, instead of hardcoding this value, allow to pass this information as
additional optional parameter 'entires-max'.
By this, creation of the archive with directories containing a large number of
entries is possible.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>