module-transaction: Add support for setting/getting module data

Module data is data associated with a module handle that is available
for the whole module loading time so it can be used also during
different operations.

We use cgo handles to preserve the life of the go objects so any value
can be associated with a pam transaction.

cmd/pam-moduler/tests/integration-tester-module/integration-tester-module.go

@@ -55,8 +55,12 @@ func (m *integrationTesterModule) handleRequest(authReq *authRequest, r *Request
 	}
 
 	var args []reflect.Value
-	for _, arg := range r.ActionArgs {
-		args = append(args, reflect.ValueOf(arg))
+	for i, arg := range r.ActionArgs {
+		if arg == nil {
+			args = append(args, reflect.Zero(method.Type().In(i)))
+		} else {
+			args = append(args, reflect.ValueOf(arg))
+		}
 	}
 
 	res = &Result{Action: "return"}