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_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package sandbox
+
+import (
+	"testing"
+)
+
+// TestCheckSupport is here for debugging purposes and has no assertions. You can quickly test
+// sandboxing support with various systems by compiling the test executable and running this test on
+// your target systems. For example, with docker, executed from the root of the repo:
+//
+// docker run --rm -it -w /go/src/github.com/mattermost/mattermost-server
+//     -v $(pwd):/go/src/github.com/mattermost/mattermost-server golang:1.9
+//     go test -c ./plugin/rpcplugin
+//
+// docker run --rm -it --privileged -w /opt/mattermost
+//     -v $(pwd):/opt/mattermost centos:6
+//     ./rpcplugin.test --test.v --test.run TestCheckSupport
+func TestCheckSupport(t *testing.T) {
+	if err := CheckSupport(); err != nil {
+		t.Log(err.Error())
+	}
+}