NFS4SERVER improvements to the wording and more defensive coding

and fixing typo

usr/share/rear/restore/NFS4SERVER/default/400_restore_with_nfs_server.sh

@@ -1,23 +1,20 @@
-# 400_restore_with_nfs_server.sh
-
-local check_file="$TARGET_FS_ROOT/$NFS4SERVER_RESTORE_FINISHED_FILE"
+local check_file="$TARGET_FS_ROOT/$NFS4SERVER_RESTORE_FINISHED_FILE" nfs_connections used_space
 
 LogPrint "Mount the nfs share: 'mount -t nfs <ip>:/ <destination>' and restore all files to mounted destination."
-LogPrint "Create the $check_file file when the restore is completed and umount the share."
+LogPrint "Create the <destination>/$NFS4SERVER_RESTORE_FINISHED_FILE file when the restore is completed and umount the share."
 
-rm -f $check_file || Error "Couldn't delete restore finished file $check_file"
+rm -f "$check_file" || Error "Couldn't delete restore finished file $check_file"
 
-LogPrint "Wait until $check_file was created and there is no connection on the NFS-Port 2049."
+LogPrint "Waiting until $check_file was created and there is no connection on the NFS-Port 2049."
 
-# or look at /var/lib/nfs/rmtab
-local nfs_connections=1
+nfs_connections=1
 while [ ! -f "$check_file" ] || [ "$nfs_connections" -gt 0 ]; do
-    local used_space
-    used_space=$(df --total --local -h --exclude-type=tmpfs --exclude-type=devtmpf | tail -n 1 | awk '{print $3}') &&
+    used_space=$(
+        df --total --local -h --exclude-type=tmpfs --exclude-type=devtmpfs | awk 'END{print $3}'
+        ) && \
         ProgressInfo "      Used storage space: $used_space"
-
     sleep 5
     nfs_connections=$(ss -tanpH state established "( sport = 2049 )" | wc -l)
 done
 
-rm -f $check_file || Error "Couldn't delete restore finished file $check_file"
+rm -f "$check_file" || Error "Couldn't delete restore finished file $check_file"