postfwd: install config from files/postfwd, idempotent /etc/default/postfwd handling

- Ship postfwd.cf and list files in files/postfwd; used by install_postfwd.sh
  and install_postfix_advanced.sh (lists only if missing, cf with backup).
- Add update_postfwd_defaults.sh.
- install_amavis.sh: disable smtpd_end_of_data_restrictions on localhost:10025.

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

files/postfwd/postfwd.bl-user

@@ -0,0 +1,8 @@
+# Gesperrte SASL-Benutzernamen; permanente REJECT-Antwort.
+# Exakter Name pro Zeile, keine Regex. Vorherige Vollfreistellungen haben Vorrang.
+#
+# Example:
+#
+#   # give SASL usernames to block here
+#   ckubu@oopen.de
+