MEDIUM: ssl: use ssl_sock_chose_sni_ctx() in the clienthello callback

MEDIUM: ssl: use ssl_sock_chose_sni_ctx() in the clienthello callback

This patch removes the code which selects the SSL certificate in the
OpenSSL Client Hello callback, to use the ssl_sock_chose_sni_ctx()
function which does the same.

The bigger part of the function which remains is the extraction of the
servername, ciphers and sigalgs, because it's done manually by parsing
the TLS extensions.

This is not supposed to change anything functionally.