README.OS400 update for new string options.

        CURLOPT_USERAGENT

        CURLOPT_USERPWD

        CURLOPT_SSH_HOST_PUBLIC_KEY_MD5

	CURLOPT_CRLFILE

	CURLOPT_ISSUERCERT

  Else it is the same as for curl_easy_setopt().

  Note that CURLOPT_ERRORBUFFER is not in the list above, since it gives the

address of an (empty) character buffer, not the address of a string.