background_process_update_7001() is broken

The update hook fails in D7, with this error:
Cannot add field .background_process: table doesn't exist.

The problem is a D6-style db_add_field() which is not compatible with D7. The first parameter should be removed from db_add_field().

Additionally, the update hook itself should be returning a string instead of an array, and there should be a comment above the hook describing the update hook itself, so it can show up in the update list.