Add monthly offsite restore tests and automatic cleanup

lib/locking.sh

@@ -11,7 +11,12 @@ acquire_backup_lock() {
       echo 'Inherited lock descriptor does not match configured lock' >&2; return 1;
     }
   else
-    exec 9>"$LOCKFILE"
+    exec 9>"$LOCKFILE" || return 1
   fi
-  flock -n 9 || { echo "Another backup/restore/update holds $LOCKFILE" >&2; return 1; }
+  local lock_code
+  flock -E "${1:-1}" -n 9 || {
+    lock_code=$?
+    echo "Another backup/restore/update holds $LOCKFILE, or the lock is unavailable" >&2
+    return "$lock_code"
+  }
 }