Change 'trusted applications' to 'privileged applications'.

The word 'trust' can be misused and can have multiple meanings [1].  The word 'privileged'  describes the concept in less ambiguous terms.

I have also reworded the informative section of the previous 'trusted applications' section in a way that (in my opinion) better motivates the need for this class of apps.  I think the 'use cases' part needs further rewording in a later pull request.

Some additional improvements will be needed to the 'security considerations'
section, but I think this needs a more thorough bit of editing.

[1] Dieter Gollmann, Why Trust is Bad for Security, Electronic Notes in Theoretical Computer Science, Volume 157, Issue 3, 25 May 2006, Pages 3-9, ISSN 1571-0661, 10.1016/j.entcs.2005.09.044 - http://www.sciencedirect.com/science/article/pii/S1571066106002891