Local tasks should automatically have a default destination query string

Steps to reproduce:
1. Sign in as an administrator
2. Go to admin/people
3. Click the 'Add user' local action
4. Fill out the form to add a new user.
5. You get 'stuck' on the Add user page, when most people would have wanted to return to the People listing.

We should automatically apply drupal_get_destination() to local actions so that the user is 'returned' to the page from where the click on the action link.