Feature request: API to obtain the results of the SSL/TLS negociation

[informatimago]

For some applications, the client code may need information about the SSL/TLS negociation which is normally obtained from or with the TLS context.

Typically may be needed:

• the server certificate,
• the server public key,
• the validation status (client / server, with possibly the CA chain that validated them).

Since drakma uses various implementations for SSL/TLS, it would be nice if it provided an API to obtain the information that is available from the TLS context, notably:

• get-ssl-peer-certificate (generic function)
• get-ssl-verify-result (generic function)

(eg. see 11.8 in: https://franz.com/support/documentation/9.0/doc/socket.htm#ssl-1)

This issue is linked to #115

I would suggest to add two opaque objects in the result values of http-request one for the socket information, and one (optional) for this security information, with the reader API to query them.