We now have a complete union fs.  It seems to work, modulo one obscure
and non-fatal weirdness.


The main addition to the new code is to add `whiteouts'.  These are
entries in the upper layer's directory which cause a file in the lower
layer to appear to be gone.  This allows removing a file that's in the
lower layer.  (The lower layer is considered read-only.)

Note that whiteouts are only implemented in FFS, LFS, and MFS, so if
you use another file system type as the upper layer, you won't be able
to delete files that are in the lower layer.


I've installed appropriate changes in the file system utilities so
that this should all work.

There are two new user-visible `features':

1) ls(1) now has a `-W' option, to show whiteouts.  Combined with the
`-F' option, whiteouts will have a `%' appended to their names.

2) rm(1) now has a `-W' option, which removes whiteouts, using the new
(and probably poorly named) undelete(2).


If you notice any problems, let me know.