GHA: add shellcheck job and fix warnings, shell tidy-ups

Reviewed-by: Daniel Stenberg
Closes #13307

.github/workflows/shellcheck.yml

@@ -0,0 +1,28 @@
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+name: shellcheck
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+permissions: {}
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+      - name: 'shellcheck'
+        run: .github/scripts/shellcheck.sh