dir/dircache: fix/simplify Access file caching
dir/dircache: fix/simplify Access file caching

Previously, WhichAccesses were remembered only for the target file and sometimes
its directory. Now we only save it for the directory. When looking up a cached
answer for WhichAccess, we now walk up the tree looking for a relevant Access file,
not just looing for one cached for the target file or its directory.

Change-Id: I9e10aeb1fcb349d9c0749455250b6087b0f442df
Reviewed-on: https://upspin-review.googlesource.com/10240
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin