Debian & Ubuntu: chmod 700 /root

"The /root dataset is created with default world-readable permissions
 (755).  debootstrap would create /root with 700 permissions, but it
 does not change the permissions if it already exists."

Reported-by: zxyrepf
Closes #115

docs/Getting Started/Debian/Debian Buster Root on ZFS.rst

@@ -433,6 +433,7 @@ Step 3: System Installation
 
      zfs create                                 rpool/home
      zfs create -o mountpoint=/root             rpool/home/root
+     chmod 700 /mnt/root
      zfs create -o canmount=off                 rpool/var
      zfs create -o canmount=off                 rpool/var/lib
      zfs create                                 rpool/var/log

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -610,6 +610,7 @@ Step 3: System Installation
      zfs create -o com.ubuntu.zsys:bootfs-datasets=rpool/ROOT/ubuntu_$UUID \
          -o canmount=on -o mountpoint=/root \
          rpool/USERDATA/root_$UUID
+     chmod 700 /mnt/root
 
    For a mirror or raidz topology, create a dataset for ``/boot/grub``::