Force collapse non-active trail items

use case:
my main menu is dropdown which requires all items to be set to expanded
I want to re-use part of this menu in a menu block, but in the block, I only want the active trail expanded.

The attached patch updates menu_block.admin.inc to add a "collapse all items" checkbox to the block settings and updates menu_block.module to check whether the user has checked "collapse all items" and respond accordingly (by ignoring the "expanded" setting of the menu items themselves).