forget: Add --unsafe-allow-remove-all option

To prevent accidentally wiping all snapshots from a repository, that
option can only be used if either a snapshot filter or a keep policy is
specified.

Essentially, the option allows `forget --tag something
--unsafe-allow-remove-all` calls to remove all snapshots with a specific
tag.

internal/restic/snapshot_policy.go

@@ -94,7 +94,11 @@ func (e ExpirePolicy) String() (s string) {
 		s += fmt.Sprintf("all snapshots within %s of the newest", e.Within)
 	}
 
-	s = "keep " + s
+	if s == "" {
+		s = "remove"
+	} else {
+		s = "keep " + s
+	}
 
 	return s
 }
@@ -186,16 +190,6 @@ func ApplyPolicy(list Snapshots, p ExpirePolicy) (keep, remove Snapshots, reason
 	// sort newest snapshots first
 	sort.Stable(list)
 
-	if p.Empty() {
-		for _, sn := range list {
-			reasons = append(reasons, KeepReason{
-				Snapshot: sn,
-				Matches:  []string{"policy is empty"},
-			})
-		}
-		return list, remove, reasons
-	}
-
 	if len(list) == 0 {
 		return list, nil, nil
 	}