Remove pointless duplication in setting MOUNT_OPTIONS and DO_MOUNT.
Try to workaround the problem that /etc/krb5.keytab
might have been moved to /etc/krb5.keytab.bu, and
then if mount fails or something, logins to ltsp server
will not work.  ---> change /etc/krb5.keytab handling
to something safer.
rename is_nfs_mounted() to is_mounted() to match
semantics better.