Use NULL's instead of Zero's

[asafo]

All over, there is usage of 0 where it should be NULL (at least DB Schema wise), examples:

1. Usage of course_categories->parent = 0 to symol top level category
2. Usage of user = 0 when there is no applicable user
3. Usage of grouping id = 0 instead on NULL (in course_modules for examaple)

This doesn't work with foreign keys, unless there would be category 0 and user with id=0,
a simplest solution, closer to the current situation, is to use NULL's when we mean 'no value'

This patch replaces the usage of 0 by use of NULL where appropriate

Dedicated branch: MOODLE_29_FOREIGNKEY_USE_NULL_4_ZERO