#14 Helpful header information for CGIs to use

Given that CGIs are required to construct their own complete HTTP headers, it would be nice if MacHTTP were to present some extra information about itself and the current transfer to the CGI, for the CGI to put into the header.

For example:
* HTTP version in use (aevt param ID 'CGhv'). For now, this will always return HTTP/1.0 as MacHTTP is presently a 1.0 server.
* Server ID string (aevt param ID 'CGsv') - e.g. "MacHTTP/2.6" - for a bit of coherency (oh, and I can display this proudly in CGI-driven virtual directory listings =)
* Header lines that the server feels useful, e.g.
- Connection: close
- MIME-Version: ...
- Message-ID: ...
(still not sure why we need a message ID for a web page - no other server does this...) All of this would go in a single item, 'CGxi'.