Fix non-constant format string in calls to Printf-like functions

In a few places, we call Printf-like functions, but for the format we
use either non-format messages (which is not tidy, but okay), or
variable messages (which can be problematic if they contain %-format
directives).

The patch fixes the calls by either moving to Print-like functions, or
using `Printf("%s", message)` instead.

These were found by a combination of `go vet` (which complains about
"non-constant format string in call"), and manual inspection.

internal/maillog/maillog.go

@@ -97,8 +97,8 @@ func (l *Logger) Auth(netAddr net.Addr, user string, successful bool) {
 		res = "failed"
 	}
 	msg := fmt.Sprintf("%s auth %s for %s\n", netAddr, res, user)
-	l.printf(msg)
-	authLog.Debugf(msg)
+	l.printf("%s", msg)
+	authLog.Debugf("%s", msg)
 }
 
 // Rejected logs that we've rejected an email.