MEDIUM: ssl: split ssl_sock_add_cert_sni()

MEDIUM: ssl: split ssl_sock_add_cert_sni()

In order to allow the creation of sni_ctx in runtime, we need to split
the function to allow rollback.

We need to be able to allocate all sni_ctxs required before inserting
them in case we need to rollback if we didn't succeed the allocation.

The function was splitted in 2 parts.

The first one ckch_inst_add_cert_sni() allocates a struct sni_ctx, fill
it with the right data and insert it in the ckch_inst's list of sni_ctx.

The second will take every sni_ctx in the ckch_inst and insert them in
the bind_conf's sni tree.