Add support for proxying SMTP messages

Other “nginx_modules_mail” USE_EXPAND flag values

Use FlagDescription
nginx_modules_mail_imapAdd support for proxying IMAP messages
nginx_modules_mail_pop3Add support for proxying POP3 messages
nginx_modules_mail_smtpAdd support for proxying SMTP messages
nginx_modules_mail_sslAdd SSL/TLS protocol support to a mail proxy server (implicit TLS, STARTTLS, etc.)

All packages providing a “nginx_modules_mail_smtp” USE flag (1)