This forces the xattr tests to properly test the behavior of ListXAttr with a zero-length buffer

This fixes getfattr(1) which otherwise fails with ERANGE.

This will catch potential data loss when the underlying storage is
full or faulty.

Writing the notification must be synchronized with closing the FUSE
connection.

6g has changed its baseaddress. Users that require small inode numbers
should just use portableHandleMap.