Changes from raven at themaw.net to support NFSv3 for automounted

Changes from raven at themaw.net to support NFSv3 for automounted
toplevel filesystems presented to the list in May. A few bug fixes
and integration changes by me. The code now works well enough to
mount the filesystem with NFSv3 and automount children but readdir
does not work yet (so no listing of automounted filesystems). The
new code is not enabled by default. To enable use auto_nfs_version = 3
in your configuration file.