MINOR: ssl: rework add cert chain to CTX to be libssl independent

MINOR: ssl: rework add cert chain to CTX to be libssl independent

SSL_CTX_set1_chain is used for openssl >= 1.0.2 and a loop with
SSL_CTX_add_extra_chain_cert for openssl < 1.0.2.
SSL_CTX_add_extra_chain_cert exist with openssl >= 1.0.2 and can be
used for all openssl version (is new name is SSL_CTX_add0_chain_cert).
This patch use SSL_CTX_add_extra_chain_cert to remove any #ifdef for
compatibilty. In addition sk_X509_num()/sk_X509_value() replace
sk_X509_shift() to extract CA from chain, as it is used in others part
of the code.