Linux-specific Posix ACL support

This adds preliminary support to Posix ACL stored on-disk as extended attributes.
TODO List:
* Fix possible SA bug. (Details in first comment)
* Add property to enable/disable this support to Posix ACLs.
* Add autoconf checks for older kernel (tested on 3.11)
* Verify behavior with user namespaces, if needed.

Signed-off-by: Massimo Maggi<me@massimo-maggi.eu>