SSL_REQUIRE_CERTIFICATE option. Value 2 has always been the default.
New Value 3 is appropriate for servers that want to re-request, but
still not require, client-auth from a client with whom an SSL session
is already established.
git-svn-id: svn://10.0.0.236/trunk@123859 18797224-902f-48f8-a5cc-f745e15eee43