This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.
In some cases, the identifiers were un-exported after noticing they had
no external users.
Besides improving documentation, it also reduces the linter noise
significantly.
This patch introduces a new "domaininfo" package, which implements a
database with information about domains. In particular, it tracks
incoming and outgoing security levels.
That information is used in incoming and outgoing SMTP to prevent
downgrades.
