set FTP kerberos security level
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_KRBLEVEL, char *level);
Pass a char * as parameter. Set the kerberos security level for FTP; this also
enables kerberos awareness. This is a string that should match one of the
following: 'clear', 'safe', 'confidential' or 'private'. If the
string is set but doesn't match one of these, 'private' will be used. Set the
string to NULL to disable kerberos support for FTP.
This option was known as CURLOPT_KRB4LEVEL up to 7.16.3
Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
CURLE_OUT_OF_MEMORY if there was insufficient heap space.