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.