This option allow to disable completely the ocsp-update.
To achieve this, the ocsp-update.mode global keyword don't rely anymore
on SSL_SOCK_OCSP_UPDATE_OFF during parsing to call
ssl_create_ocsp_update_task().
Instead, we will inherit the SSL_SOCK_OCSP_UPDATE_* value from
ocsp-update.mode for each certificate which does not specify its own
mode.
To disable completely the ocsp without editing all crt entries,
ocsp-update.disable is used instead of "ocsp-update.mode" which is now
only used as the default value for crt.
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