implemented client side SMB2 signing

implemented client side SMB2 signing

This doessn't work against Windows yet, and I've submitted a WSPP
request for clarification of the docs to try and find out
why. Meanwhile this is no worse than what we had, as it only gets used
when the server demands signing, and we didn't work then anyway.
(This used to be commit b788096add3586d7277efcd3bf5ca7f3a604cb7a)