drush fserver-distro and drush v.5 compatibility

The usage of drush fserver-distro won't work, if you use Drush version >=5,
because drush_backend_invoke() was replaced with drush_invoke_process()
as the preferred way for a Drush command to call another Drush command in Drush v.5.

@see http://www.drush.org/drush-4-to-drush-5

The attached patches...

1. fserver-drush-fserver-distro-v4-to-v5.patch converts the fserver-distro command from Drush v.4 to v.5
2. fserver-drush-fserver-distro-v4-to-v5-switch.patch provides a version switch and let you work with v.5 and below versions