Support Kerberos on Macs

Box currently supports NTLM V2 on Windows Box Sync, but only support NTLM v1 (with a keychain entry) on Mac Box Sync. Additionally, Kerberos is supported in the login flow (via the embedded browser) but not for the actual network requests that Box Sync makes.

As such, some organizations would like Box Sync to fully support Kerberos as an authentication method for the login process AND for all network requests.

Request: Add full authentication support for Kerberos.