Fix a couple of NFS client side caching snafus. First, we have to update

Fix a couple of NFS client side caching snafus.  First, we have to update
np->n_mtime if the file has not being modified locally, otherwise n_mtime
becomes stale and both the dynamic cache timeout calculation breaks and
stat() calls checking for the last modified time break.  This was causing
indeterminant operation with make for obvious reasons.

The Second problem was that we were not checking for an attrstamp of 0
(indicating a timed out entry), resulting in another incorrect timeout
calculation.