refactor: protect => quic.sockopts

Android's VpnService.protect() itself is confusing, so we rename the
"protect" feature with the name `fdControlUnixSocket` and make it a
sub-option under `quic.sockopts`.

A unit test is added to make sure the protect feature works.

I also added two other common options to `quic.sockopts` that I copied
from my other projects but did not fully test here.

app/internal/sockopts/sockopts_linux_test.go

@@ -0,0 +1,53 @@
+//go:build linux
+
+package sockopts
+
+import (
+	"net"
+	"os"
+	"os/exec"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/sys/unix"
+)
+
+func Test_fdControlUnixSocketImpl(t *testing.T) {
+	sockPath := "./fd_control_unix_socket_test.sock"
+	defer os.Remove(sockPath)
+
+	// Run test server
+	cmd := exec.Command("python", "fd_control_unix_socket_test.py", sockPath)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	err := cmd.Start()
+	if !assert.NoError(t, err) {
+		return
+	}
+	defer cmd.Process.Kill()
+
+	// Wait for the server to start
+	time.Sleep(1 * time.Second)
+
+	so := SocketOptions{
+		FdControlUnixSocket: &sockPath,
+	}
+	conn, err := so.ListenUDP()
+	if !assert.NoError(t, err) {
+		return
+	}
+	defer conn.Close()
+
+	err = controlUDPConn(conn.(*net.UDPConn), func(fd int) (err error) {
+		rcvbuf, err := unix.GetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_RCVBUF)
+		if err != nil {
+			return
+		}
+		// The test server called setsockopt(fd, SOL_SOCKET, SO_RCVBUF, 2500),
+		// and kernel will double this value for getsockopt().
+		assert.Equal(t, 5000, rcvbuf)
+		return
+	})
+	assert.NoError(t, err)
+}