New smtptls-finger utility.

This makes it much easier to test DANE, STARTTLS, etc, without
reconfiguring real mail flows.

Question: is the name smtptls-finger OK?
Question: is it OK to add a new file to $command_directory/ whose name
	  does not start with "post".
Question: Is it OK to install a mostly crippled version when TLS is
          disabled?  Without TLS all it can do is report the banner
	  and EHLO reply.

Note: The src/ sub-directory is the abbreviated name "tlsfinger",
but that seemed to generic a name to associate with an SMTP-only
utility.