VFS messaging/interfacing work stage 7d/99: More firming up of stage 7.

VFS messaging/interfacing work stage 7d/99: More firming up of stage 7.

Additional work to deal with old-api/new-api issues.  Cut more stuff
out of the old-api's cache_enter() routine to deal with deadlocks, at
the cost of some performance loss (temporary until the VFS's start using
the new APIs).  Change UFS and NFS to not purge whole directories in
*_rename() and *_rmdir().

Add some minor breakage to the API which will not be fixed until the VFS's
get new rename implementations - renaming a directory in which a process
has chdir'd will create problems for that process.  This doesn't happen
normally anyway so this temporary breakage should not cause any significant
problems.

Bug-reports-by: walt, Sascha Wildner, others