Add the capability to set "smb encrypt = required"

Add the capability to set "smb encrypt = required"
on a share (or global) and have the server reply with
ACCESS_DENIED for all non-encrypted traffic (except
that used to query encryption requirements and set
encryption state).
Jeremy.
(This used to be commit d241bfa57729bb934ada6beabf842a2ca7b4f8a2)