s3-winbindd Only use SamLogonEx when we can get unencrypted session keys

s3-winbindd Only use SamLogonEx when we can get unencrypted session keys

This ensures that we have some check on the session keys being returned
as the RC4 cipher is not checksumed.

The check comes from the fact that the credentials chain is tied to
the session key, and so if the credentials check passes then the
netlogon session key will be correct, and so the user session key
will be correctly decrypted.

Andrew Bartlett

Part of a fix for bug #8599 (WINBINDD_PAM_AUTH_CRAP returns invalid user session
key).