Implement vfs_ea_tdb

Implement vfs_ea_tdb

This is an option for file systems that do not implement xattrs: in
lockdir/eas.tdb an array of xatts per inode is stored.

It can not solve the problem that xattrs might reappear if a posix-level
process deletes a file and happens to re-create it under the same name. On file
systems with birthtime we might have a chance to detect this, but not with
standard posix. A future version might put relief on file systems that do have
xattrs but where these are severely limited in size/speed/whatever: We can put
a simple marker as a native xattr, but the xattrs proper are stored in the tdb.

Volker
(This used to be commit 2036b4c5ad677b8a477b34b0f076febab0abff5e)