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/ipc_test.go

@@ -9,6 +9,8 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost-server/plugin/rpcplugin/rpcplugintest"
 )
 
 func TestIPC(t *testing.T) {
@@ -17,7 +19,7 @@ func TestIPC(t *testing.T) {
 	defer os.RemoveAll(dir)
 
 	pingpong := filepath.Join(dir, "pingpong.exe")
-	compileGo(t, `
+	rpcplugintest.CompileGo(t, `
 		package main
 
 		import (