GH-71 Add TLSEXT_TYPE_ prefixed constants.

TLSEXT_TYPE_ constants map TLS ExtensionType names used by OpenSSL to their
numerical values.

These constants are used with, for example, SSL_client_hello_get0_ext().

IANA registry for Transport Layer Security (TLS) Extensions is at:
https://www.iana.org/assignments/tls-extensiontype-values/

Author: h-vn