Custom EHLO value for SMTP checks

[Val V]

Putting localhost is not really the best practice and many production-grade SMTP servers don’t accept it.

Right now from what I see it’s hardcoded in smtp.c, but it’d be good to have a parameter to customize it like in openssl.

Obviously a custom TCP I/O check can be used instead (only with SMTPS and not STARTTLS, see #1041 for details), but it seems low effort to make that value configurable instead.