When called as automount -u, try using umount(8) before unmount(3). When an
NFS mount is being unmounted, umount(8) notifies the mountd server and if
successful, will also remove the associated entry from /var/db/mounttab.
This uses auto_popen() to call umount(8) in the same fashion that
automountd(8) uses auto_popen() to call mount(8) when it mounts the filesystem.