implement outgoing dmarc aggregate reporting

in smtpserver, we store dmarc evaluations (under the right conditions).
in dmarcdb, we periodically (hourly) send dmarc reports if there are
evaluations. for failed deliveries, we deliver the dsn quietly to a submailbox
of the postmaster mailbox.

this is on by default, but can be disabled in mox.conf.

store/cleanuptemp.go

@@ -0,0 +1,17 @@
+package store
+
+import (
+	"os"
+
+	"github.com/mjl-/mox/mlog"
+)
+
+// CloseRemoveTempFile closes and removes f, a file described by descr. Often
+// used in a defer after creating a temporary file.
+func CloseRemoveTempFile(log *mlog.Log, f *os.File, descr string) {
+	name := f.Name()
+	err := f.Close()
+	log.Check(err, "closing temporary file", mlog.Field("kind", descr))
+	err = os.Remove(name)
+	log.Check(err, "removing temporary file", mlog.Field("kind", descr))
+}