Fix bugs in custom extensions handling

 1) tlsext_custom_types wasn't getting cleared prior to renegotiated handshakes, causing the client's custom extensions to be erroneously detected as duplicates (reported by Scott Deboy)

 2) SSLv2 client hello should be disabled when client custom extensions are used (reported by Scott Deboy)

 3) SSL_CTX_use_certificate() was clearing the serverinfo state, causing httpd (for example) to ignore the serverinfo.