postfix: SMTP Smuggling Protection

(Patch sent by oxzi in libera/#nixos-mailserver, who does not wish to create a GitLab account.)

Enable Postfix SMTP Smuggling protection, introduced in Postfix 3.8.4, which is, currently, only available within the nixpkgs' master branch.

• https://github.com/NixOS/nixpkgs/pull/276104
• https://github.com/NixOS/nixpkgs/pull/276264

For information about SMTP Smuggling:

• https://www.postfix.org/smtp-smuggling.html
• https://www.postfix.org/postconf.5.html#smtpd_forbid_bare_newline