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/sandbox/sandbox_other.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// +build !linux
+
+package sandbox
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/mattermost/mattermost-server/plugin/rpcplugin"
+)
+
+func newProcess(ctx context.Context, config *Configuration, path string) (rpcplugin.Process, io.ReadWriteCloser, error) {
+	return nil, nil, checkSupport()
+}
+
+func checkSupport() error {
+	return fmt.Errorf("sandboxing is not supported on this platform")
+}