Add set_client_SNI_hostname to specify client-side SNI hostname.

No attempt is done at checking SSL_set_tlsext_host_name(SSL_ctrl)'s return
value because I could not find it documented anywhere.

I have verified that the `SSL_set_tlsext_host_name` macro exists in both OpenSSL 0.9.8
(since 2007, see https://github.com/openssl/openssl/commit/865a90eb4f0b0e3abbdd9dc2d3a4d57595575315 ) and >1.0 branches, so adding it poses
 no compatibility problems.