Validate server certificates without bumping

Validate server certificates without bumping

This patch add support for the "Validate server certificates without bumping"
use case described on the Peek and Splice wiki page:
    http://wiki.squid-cache.org/Features/SslPeekAndSplice

This patch send to the certificate validation helper the  certificates and
errors found in SslBump3 step, even if the splicing mode selected.
In the case the validation helper found errors in certificates an error
page returned to the http client.

The SSL error forwarding is controlled by ACLs along these lines:

   sslproxy_cert_error allow sslBoringErrors
   sslproxy_cert_error allow serversWithInvalidCerts
   sslproxy_cert_error deny all

This is a Measurement Factory project