Modify sharenfs=on default behavior

Modify sharenfs=on default behavior

While it may sometimes be convenient to export an NFS filesystem with
no_root_squash it should not be the default behavior.  Align the
default behavior with the Linux NFS server defaults.  To restore
the previous behavior use 'zfs set sharenfs="no_root_squash,..."'.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9397
Closes #9425