Update and document syntax for smarthost string in smtpd.conf:

Update and document syntax for smarthost string in smtpd.conf:

- the +auth specifier is removed: it is implied by the presence of an
  auth label in the rest of the string
- secure:// is removed: use smtp+tls:// or smtps://
- tls:// is replaced by smtp+tls://
- smtp:// becomes SMTP with opportunistic STARTTLS
- smtp+tls:// becomes SMTP with mandatory STARTTLS

Adjust your config file accordingly.

ok gilles@