MEDIUM: ocsp: fix ocsp when the chain is loaded from 'issuers-chain-path'

MEDIUM: ocsp: fix ocsp when the chain is loaded from 'issuers-chain-path'

This fixes OCSP, when issuer chain is in a separate PEM file. This is a
case of issuers-chain-path keyword, which points to folder that contains only
PEM with RootCA and IntermediateCA.

Before this patch, the chain from 'issuers-chain-path' was applied
directly to the SSL_CTX without being applied to the data->chain
structure. This would work for SSL traffic, but every tests done with
data->chain would fail, OCSP included, because the chain would be NULL.

This patch moves the loading of the chain from
ssl_sock_load_cert_chain(), which is the function that applies the chain
to the SSL_CTX, to ssl_sock_load_pem_into_ckch() which is the function
that loads the files into the ckch_data structure.

Fixes issue #2635 but it changes thing on the CLI, so that's not
backportable.