RemoteCommandProxy

Create a placeholder proxy command to represent an unknown command.

We use these only when executing remote commands that do not exist locally. We will let the remote end decide whether these will be "command not found," or some other behavior, as the remote end might have additional functionality installed.

Also note that, for remote commands, we create the proxy command prior to attempting to bootstrap Drupal further, so the proxy command may be used in place of some command name that is available only for Drupal sites (e.g. pm:list and friends, etc.).