ci: add shellcheck job and script

Add FIXME for OS/400 scripts.

Cherry-picked from #1358

ci/shellcheck.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+# FIXME: os400/* scripts
+
+shellcheck --version
+# shellcheck disable=SC2046
+shellcheck --exclude=1091 \
+  --enable=avoid-nullary-conditions,deprecate-which \
+  $(grep -l -E '^#!(/usr/bin/env bash|/bin/sh|/bin/bash)' $(git ls-files | grep -v -F 'os400/'))