Tweaks to the introduction

Try to frame things in a more generic "security" sense, instead of
always talking about encryption.

Mention a desire to eventually increase the security baseline.

Attempt to reword portions for clarity.

Attempt to provide examples of how opportunistic authentication
may be performed, as well as opportunistic encryption.
Adjust definitions

Define an "authenticated connection", and define authenticated
and unauthenticated encryption in terms of that.
Another place to favor security over encryption
More protocol vs. pattern clarifications
Capabilities determination is per-protocol

... and should be covered in each protocol's spec.  DANE is nice, though.
Rework the design pattern section for clarity and concision