Provide ssl_version_string() function, which uses one of those modern C

Provide ssl_version_string() function, which uses one of those modern C
constructs (a switch statement) and returns the appropriate string defined
by SSL_TXT_* for the given version, including support for DTLSv1 and
DTLSv1-bad. Use this function in SSL_get_version() and SSL_SESSION_print().

ok beck@