GH646: Update help for s_server command.

    * added missing help option messages
    * ecdh_single option is removed as it is a no-op and not an option
    supported in earlier versions
    * ssl_ctx_security_debug() was invoked before ctx check for NULL
    * trusted_first option can be removed, as it is always enabled in 1.1.
    But not removed the option, require confirmation.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>