Tests: added has_feature() tests for IO::Socket::SSL. The following distinct features supported: - "socket_ssl", which requires IO::Socket::SSL and also implies existance of the IO::Socket::SSL::SSL_VERIFY_NONE() symbol. It is used by most of the tests. - "socket_ssl_sni", which requires IO::Socket::SSL with the can_client_sni() function (1.84), and SNI support available in Net::SSLeay and the OpenSSL library being used. Used by ssl_sni.t, ssl_sni_sessions.t, stream_ssl_preread.t. Additional Net::SSLeay testing is believed to be unneeded and was removed. - "socket_ssl_alpn", which requires IO::Socket::SSL with ALPN support (2.009), and ALPN support in Net::SSLeay and the OpenSSL library being used. Used by h2_ssl.t, h2_ssl_verify_client.t, stream_ssl_alpn.t, stream_ssl_preread_alpn.t. - "socket_ssl_sslversion", which requires IO::Socket::SSL with the get_sslversion() and get_sslversion_int() methods (1.964). Used by mail_imap_ssl.t. - "socket_ssl_reused", which requires IO::Socket::SSL with the get_session_reused() method (2.057). To be used in the following patches. This makes it possible to simplify and unify various SSL tests.