from-header-binding: what the reviews had left as found, and address extensions

The owner on the three things the reviews' commit left as found: can they be
fixed? They can.

- Downtime that crossed a UTC midnight was all booked on the day of the
  restart: it is cut at the midnights between (byDay).
- A message with the null envelope sender - a bounce an application writes
  itself - left a refusal row without a domain, which no domain
  administrator would see: the row takes the domain the login is bound to.
- A log read again after an unclean stop counted its deferrals again: while
  the tailer catches up, a warning no later than the last one counted is
  passed over. The mark is kept to the fraction of a second the log gives.

And on the check being stricter than Postfix for an address with an
extension (invoices+x@ passed in the envelope, was refused in the header):
it makes Postfix's second try now, the address without its extension.
recipient_delimiter was the distribution's default; it is set in
postfix-config.sh, since the check depends on it. e2e sends one through.

Test server, this tree: go vet ok, go test 23 packages ok, make e2e ok (339 s).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/sender_check_test.go

@@ -40,6 +40,7 @@ func testSenderCheck(t *testing.T, sc *scenario) {
 		{"own address", wildcard, "alerts@" + senderDomain, true},
 		{"another address of the domain, with a name", wildcard, `"The Shop" <orders@` + senderDomain + `>`, true},
 		{"the listed address", list, "billing@" + senderDomain, true},
+		{"the listed address with an extension, in envelope and header", who{listLogin, listPassword, "billing+march@" + senderDomain}, "billing+march@" + senderDomain, true},
 		// What a mail library does with a long display name: the line is
 		// folded, and Postfix hands the milter both of its lines.
 		{"a long name, folded onto a second line", wildcard, "\"The Shop, its orders and its returns desk\"\r\n <orders@" + senderDomain + ">", true},