Introduce CI support for build checks.

Utilize GitHub actions to verify the contents of the repository.
This is implemented through a test matrix of 13.4 and 14.1 releases
where the `net/wifibox-alpine` and `net/wifibox-core` ports are
tested for build and package list issues.

.github/workflows/checks.yml

@@ -0,0 +1,37 @@
+name: Build Checks
+on: [push]
+
+jobs:
+  checks:
+    strategy:
+      matrix:
+        version: ["13.4", "14.1"]
+    runs-on: ubuntu-latest
+    name: Continuous Integration
+    steps:
+    - uses: actions/checkout@v4
+    - name: "FreeBSD ${{ matrix.version }}"
+      id: port_checks
+      uses: vmactions/freebsd-vm@v1
+      with:
+        release: "${{ matrix.version }}"
+        usesh: true
+        prepare: |
+          pkg install -q -y gitup portconfig
+          pkg install -q -y gtar patchelf squashfs-tools-ng grub2-bhyve socat
+          gitup ports -v0
+
+        run: |
+          set -eu
+          kldload linux64
+          mkdir -p /compat/linux
+
+          make -C net/wifibox-alpine check-plist
+          make -C net/wifibox-alpine install
+          make -C net/wifibox-alpine deinstall
+          make -C net/wifibox-alpine clean
+
+          make -C net/wifibox-core check-plist
+          make -C net/wifibox-core install
+          make -C net/wifibox-core deinstall
+          make -C net/wifibox-core clean