Set passphrase to private key
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_KEYPASSWD, char *pwd);
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3, SMTPS etc.
This option was known as CURLOPT_SSLKEYPASSWD up to 7.16.4 and CURLOPT_SSLCERTPASSWD up to 7.9.2.
Returns CURLE_OK if TLS enabled, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.