ABC-22: Plugin sandboxing for linux/amd64 (#8068)

* plugin sandboxing

* remove unused type

* better symlink handling, better remounting, better test, whitespace
fixes, and comment on the remounting

* fix test compile error

* big simplification for getting mount flags

* mask statfs flags to the ones we're interested in

plugin/rpcplugin/rpcplugintest/rpcplugintest.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package rpcplugintest
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func CompileGo(t *testing.T, sourceCode, outputPath string) {
+	dir, err := ioutil.TempDir(".", "")
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+	require.NoError(t, ioutil.WriteFile(filepath.Join(dir, "main.go"), []byte(sourceCode), 0600))
+	cmd := exec.Command("go", "build", "-o", outputPath, "main.go")
+	cmd.Dir = dir
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	require.NoError(t, cmd.Run())
+}