Remove trailing whitespace

Additionally, added a GitHub Action to ensure no more trailing
whitespace will creep in again in the future.

I'm excluding Markdown files from this check, since trailing whitespace
is significant there, and also excluding `build-aux/install-sh` because
there is significant trailing whitespace on the line that sets
`defaultIFS`.

.github/workflows/check_formatting.yaml

@@ -0,0 +1,10 @@
+name: 'Check Formatting'
+on: [pull_request]
+jobs:
+  check-formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v3
+      - name: Check for trailing whitespace
+        run: scripts/check_trailing_whitespace.sh