Fix erroneous SSL certificate warnings

If you attempted to use SSL to connect to a server that speaks
STARTTLS, you should get an SSL protocol error.  Instead, you
were likely to get an "Unrecognized Certificate" error that shows
you an unrelated certificate chain and asks you to accept it or
reject it.  Neither action would work because the actual problem
had nothing to do with certificates.  The unrelated certificate
chain that popped up had been statically stored when validating
a prior connection to a different server.

With this patch, certificate chains are no longer stored statically
when validating server connections.

Issue 5886 is an example of a user experiencing this problem.
Merge pull request #365 from zjw/Issue_5886_misleading_ssl_errors

Fix erroneous SSL certificate warnings