Finding correct path to drush.php on other platforms

This is one of the major issues relating to multi platform support (http://drupal.org/node/278674)

It is hard coded to point to $publish_path/profiles/hostmaster/modules/drush/drush.php at present. And that works fine on a single
platform, but unless each of the platforms are hostmasters, that will not work.

With the (http://drupal.org/node/321949) we will have a headless install profile, which would make it's location either profiles/hostmaster or profiles/hostslave , which makes it a bit simpler. I'm thinking we should build in the platform type
functionality for this (very simply for now), in that the platform type you are connecting to changes the command that
you actually execute.

This would be useful for future expansion to other cms'. As long as they return the same thing, you wouldn't necessarily have
to call drush when communicating with a platform. It could be another cms specific script, with different parameters. But it
_must_ return the same output.