Changes to a link's categories don't stick

[dphiffer]

With trunk, try the following:

1. Add a new link without selecting a category
2. Click edit on your new link
3. Select a new category in addition to or instead of the default "Blogroll"
4. Click update

On my install it doesn't update the category status of the link.

[ryan]

(In [13053]) Test explicitly for false from insert()/update(). Fixes early return that preventing link categories from being saved. fixes #12196