update Terminal.spawn_child() to use spawn_async, not spawn_sync

terminatorlib/terminal.py

@@ -1488,13 +1488,17 @@ class Terminal(Gtk.VBox):
 
         dbg('Forking shell: "%s" with args: %s' % (shell, args))
         args.insert(0, shell)
-        result,  self.pid = self.vte.spawn_sync(Vte.PtyFlags.DEFAULT,
+        self.pid = self.vte.spawn_async(
+            Vte.PtyFlags.DEFAULT,
             self.cwd,
             args,
             envv,
             GLib.SpawnFlags.FILE_AND_ARGV_ZERO | GLib.SpawnFlags.DO_NOT_REAP_CHILD,
             None,
             None,
+            -1,
+            None,
+            None,
             None)
         self.command = shell