postfix: ETRN discarded from EHLO silently

After the 2.14.4 rollout the test server's mail.log held "discarding EHLO
keywords: ETRN" for every connection on every port: smtpd logs a discard
unless the mask has EHLO_MASK_SILENT (Postfix 3.10.13, src/smtpd/smtpd.c:2016;
src/global/ehlo_mask.c:88 "SILENT-DISCARD"; postconf(5)
smtpd_discard_ehlo_keywords, "Specify the silent-discard pseudo keyword to
prevent this action from being logged"). smtpd_discard_ehlo_keywords is now
silent-discard,etrn; ETRN is still refused (smtpd_etrn_restrictions=reject)
and still not offered.

make e2e (64 passed; checkPostfixHardening reads the value back,
testInboundRelay still sees no ETRN in EHLO and 554) and go vet + go test
(26 packages) on the test server for this tree.

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

CHANGELOG.md

@@ -5,6 +5,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Fixed
+
+- **ETRN is discarded from the EHLO reply silently**: since `2.14.4` Postfix
+  logged "discarding EHLO keywords: ETRN" for every connection on every port
+  (seen on the test server after the rollout); `silent-discard` keeps the
+  discard and drops the line.
+
 ## [2.14.4] - 2026-10-17
 
 The follow-ups of the security audit of 2026-10-17