Various fixes focused on xattr and system.* xattr namespace.
They are required to have usable ACLs managed via 9P
- 9P/xattr : bug fixed, assumption was made that xattr's
  value is a string. This prevented ACLs
   (content of system.posix_acl_access), which is a binary buffer,
   to be set properly.
- 9P/ACL: Hook added to deal properly with pseudo xattr "system.posix_acl_access"
- 9P/ACL: the protocol implements hooks to deal correctly with "system.posix_acl_access" pseudo xattr
- 9P/xattr : an ACL corner case (visible with FSAL_VFS and not FSAL_LUSTRE) was fixed.
- FSAL_VFS/xattr : code in xattrs.c has been hardened as 9P ACL support was backported to VFS
Big commit to update the FSAL_LUSTRE tree

LUSTRE/pNFS: This commit implements the support of pNFS for FSAL_LUSTTRE
CHECKPATH: work done on FSAL_LUSTRE
FSAL_LUSTRE: portability feature, do not show <lustrre_root>/.lustre
With Lustre 2.4 and higher, this object starts to behace quiet weirdly.
FSAL_LUSTRE: Fix regression (parent dir badly managed via FID) observed
in lustre > 2.4
FSAL_LUSTRE: ported to LUSTRE 2.4 and 2.5
	adding fake file lustre_errno.h to allow compilation with Lustre 2.5
FSAL_SHOOK: missing pNFS vector
FSAL_LUSTRE: a few unused/unitialised variables fixed
pNFS/LUSTRE: invalidate attrs on close (force DS/MDS coherency). Think about something more subtle.
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