Fix up some of the SMB signing code:

Fix up some of the SMB signing code:

The problem was that *all* packets were being signed, even packets before
signing was set up.  (This broke the session request).

This fixes it to be an 'opt in' measure - that is, we only attempt to sign
things after we have got a valid, non-guest session setup as per the CIFS spec.

I've not tested this against an MS server, becouse my VMware is down, but
at least it doesn't break the build farm any more.

Andrew Bartlett
(This used to be commit 1dc5a8765876c1ca822e454651f8fd4a551965e9)