CURLOPT_KRBLEVEL(3) on macOS

The text efdfc29201cd, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

set FTP kerberos security level

CURLOPT_KRBLEVEL(3) on macOS now