Switch from drupal_goto to menu_set_active_menu_item

I was having some trouble where going to user/register with a destination parameter set (user/register?destination=xxx) was causing it to go back to the home page. I have Logintoboggan 6.x-1.7 installed as well. I was able to fix the issue by replicating the pattern that the user module uses in user_page where it modifies the active menu item and recalls menu_execute_active_handler. It seems to be a more bulletproof method in the case where you don't really want to do a full redirect, but instead just want to alter what is presented to the user at that path.