Add way to query file attributes via cifs xattr

Add parsing for new pseudo-xattr user.cifs.dosattrib file attribute
so tools can recognize what kind of file it is, and verify if common
SMB3 attributes (system, hidden, archive, sparse, indexed etc.) are
set.

Signed-off-by: Steve French <steve.french@primarydata.com>
Reviewed-by: Pavel Shilovsky <pshilovsky@samba.org>