Add utility script to detect trailing whitespace

Run the script in CI as well to ensure we don't accidentally add
trailing whitespace again.

.gitlab-ci.yml

@@ -501,6 +501,7 @@ misc:
     - rm CHANGES.tmp
     - sh util/check-categories.sh
     - sh util/check-gitignore.sh
+    - sh util/check-trailing-whitespace.sh
     - if git grep SYSTEMTESTTOP -- ':!.gitlab-ci.yml'; then echo 'Please use relative paths instead of $SYSTEMTESTTOP.'; exit 1; fi
     - bash util/unused-headers.sh
     - bash util/xmllint-html.sh

util/check-trailing-whitespace.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+trailing_whitespace="$(git grep -I -n '[[:space:]]$' -- ':!*/testdata/*')"
+
+if [ -n "${trailing_whitespace}" ]; then
+	echo "The following occurrences of trailing whitespace characters were found:"
+	echo
+	echo "${trailing_whitespace}"
+	echo
+	echo "Please remove the trailing whitespace from these files."
+	exit 1
+fi