MEDIUM: ssl: load DH param in struct cert_key_and_chain

MEDIUM: ssl: load DH param in struct cert_key_and_chain

Load the DH param at the same time as the certificate, we don't need to
open the file once more and read it again. We store it in the ckch_node.

There is a minor change comparing to the previous way of loading the DH
param in a bundle. With a bundle, the DH param in a certificate file was
never loaded, it only used the global DH or the default DH, now it's
able to use the DH param from a certificate file.