Improve security and privacy section

This probably wont be good enough as-is, but it represents my first contribution to improvements to the security and privacy section of this spec.  I hope the discussion will help me improve it.

This pull request is in response to this thread: https://github.com/sysapps/telephony/issues/26

Still TODO: 
 * Validation of the threats
 * Less hand-waving around the mitigations
 * User interaction guidelines (to demonstrate expectations on the UI)
 * Linking to the Runtime spec, particularly the new "privileged" part.