GMail for instance sends us a * CAPABILITY line before the result.

Useful to see if a message ends up in the spam folder.

Also, remove the useless default from --config.

So that "run-go-test" actually works (or the cover-package
trick will cause the tests to FAIL, sigh).

Also move some common code around.

Provide our own net/smtp.Auth implementation to do AUTH PLAIN without
the TLS check.

Fixes issue #1.

Allows verifying things like DKIM signatures, spam classification,
etc. on the message as it was delivered.