Forbid backslashes in group names.

We used to only forbid them under Windows.

group/group.go

@@ -420,10 +420,14 @@ func Add(name string, desc *Description) (*Group, error) {
 }
 
 func validGroupName(name string) bool {
-	if filepath.Separator != '/' &&
+	if strings.ContainsRune(name, '\\') {
-		strings.ContainsRune(name, filepath.Separator) {
 		return false
 	}
+	if s := filepath.Separator; s != '/' && s != '\\' {
+		if strings.ContainsRune(name, s) {
+			return false
+		}
+	}
 
 	s := path.Clean("/" + name)
 	if s == "/" {

group/group_test.go

@@ -338,6 +338,7 @@ func TestValidGroupName(t *testing.T) {
 		{"foo/..", false},
 		{"foo/./bar", false},
 		{"foo/../bar", false},
+		{"foo\\bar", false},
 		{"foo", true},
 		{"foo/bar", true},
 	}