DrushMakeDownload_CVS can't do local checkouts, cleanup of duplicate code.

putting the initial colon in the -d switch breaks checkouts/exports from local repos: drush_shell_exec("(cd %s; cvs -d:%s checkout -d%s %s %s)", $this->project->tmp_path, $this->project->download['root'], $this->project->name, $flags, $this->project->download['module']);

attached patch fixes this, and also cleans up some duplicate code in that class.