helper: separate helper args fd builder from dbus

This method of passing arguments is used in bubblewrap as well as other tools, this commit separates the argument builder/writer to the helper package and generalise it as an interface.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

dbus/run.go

@@ -43,7 +43,7 @@ func (p *Proxy) Start(ready *chan bool) error {
 
 	statsP, argsP := p.statP[0], p.argsP[1]
 
-	if _, err := argsP.Write([]byte(*p.seal)); err != nil {
+	if _, err := p.seal.WriteTo(argsP); err != nil {
 		if err1 := p.cmd.Process.Kill(); err1 != nil {
 			panic(err1)
 		}