docs: Use consistent naming for Kerberos
urldata: Don't define sec_complete when no GSS-API support present

This variable is only used with HAVE_GSSAPI is defined by the FTP code
so let's place the definition with the other GSS-API based variables.
Kerberos auth: Use symbols qualified with _KERBEROS5

* Add a consistent symbol naming with _KERBEROS5 at the end
* Fix curl version reference to 7.39.1
* Add some TODOs