smtp: fix for server doesn't support starttls extension Currently we only support enabling TLS by using SMTP STARTTLS extension. But not all the servers support it. With this patch, user can choose which way to enable TLS: * Default: tls = none port = 25 * To use STARTTLS: tls = starttls port = 465 * To use SMTP over SSL: tls = smtps port = 465 To keep backward compatibility, when tls = true, we use STARTTLS to enable TLS. Signed-off-by: Zhigang Wang <w1z2g3@gmail.com>