This is a follow-up on the `go fix` run in commit
0210c5fd92.
That commit recorded the automated updates.
This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.
It also removes some now-obsolete comments and statements.
This patch contains the result of running `go fix ./...`, which applies
simplifications that make use of modern language features.
There are no logic or behaviour changes.
This patch adds a cross-tool integration check that uses
driusan/dkim's dkimverify to confirm it can verify our own DKIM signatures.
It is optional, since the tool may not be present.
Alberto Bertogli