KRB5: Handle KRB5_REALM_UNKNOWN as ERR_NETWORK_IO
    
    Resolves:
        https://fedorahosted.org/sssd/ticket/2866
    
    This would help users who authenticate to AD trust servers while offline
    and see error messages such as:
        [get_and_save_tgt] (0x0020): 996: [-1765328230][Cannot find KDC for realm "AD.EXAMPLE.COM"]
    
    in the krb5_child.log
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>