[mod] backup script rewritten (inspired and adpated from example_ynh).

Currently neither backup nor restore are useful but are safe enough to
not break the upgrade script. That's what matters for now.

scripts/upgrade

@@ -36,6 +36,13 @@ if [ -f /etc/php5/fpm/pool.d/vpnadmin.conf ]; then
 fi
 test -d /var/www/vpnadmin && mv /var/www/vpnadmin /var/www/${app}
 
+# Versions known to have a buggy backup script
+buggy_versions="1.0.0 1.0.1 1.1.0"
+curr_version=$(read_manifest version)
+if echo $buggy_versions | grep -w $curr_version > /dev/null; then
+  echo "Your current version of ${app} is very old: ${curr_version}. Please ignore the next warning." >&2
+fi
+
 #=================================================
 # BACKUP BEFORE UPGRADE THEN ACTIVE TRAP
 #=================================================