Kerberos SSO authentication

Implements basic kerberos SSO authentication.
This never succeeds for me even when FreeIPA is running:
```
$ nslookup cockpit.lan
Server:		10.111.111.100
Address:	10.111.111.100#53

*** Can't find cockpit.lan: No answer
```

This works:
$ nslookup -type=SRV _ldap._tcp.cockpit.lan
Server:		10.111.111.100
Address:	10.111.111.100#53

_ldap._tcp.cockpit.lan	service = 0 100 389 f0.cockpit.lan.
```