Bug 1042135 - Change three-state DNT back to two state and update text.  (r=dolske,jst,mcmanus)

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extra : rebase_source : 52556477cba80e87651ce7c84a531a2de500d21a
Bug 1042135 - Update Directory Links Provider to use only the dnt.enabled pref after removing the dnt.value pref. (r=adw)

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extra : rebase_source : cc511558eb39099bc745116798ca64829de79d0d