smtpd_test.go: run through gofmt -s
smtpd.go: add more documentation and minor code/doc fiddles due to AUTH

I have neurotically expanded a bunch of comments, mostly related to the
new AUTH support by Felix Lange, and added some paranoid code to handle
corner cases. I also re-wordwrapped some of the comments.

(I wrote a bunch more comments partly because I'm not familiar with either
SMTP AUTH or the new AUTH code, so I wanted to give myself a chance
to understand it in the future once my current temporary familiarity
has faded.)