Introduce CI support for build checks.

Leverage GitHub Actions to run some build checks on each new
change.  Currently this runs ShellCheck on the Wifibox script,
but it could be extended over time.

.github/workflows/checks.yml

@@ -0,0 +1,19 @@
+name: Build Checks
+on: [push]
+
+jobs:
+  checks:
+    runs-on: ubuntu-latest
+    name: Sanity check CI job
+    steps:
+    - uses: actions/checkout@v4
+    - name: ShellCheck results
+      id: shellcheck
+      uses: vmactions/freebsd-vm@v1
+      with:
+        usesh: true
+        prepare: |
+          pkg install -y hs-ShellCheck
+
+        run: |
+          make VERSION=9.9.9 shellcheck