Merge from HEAD the SMB signing patch that I developed a couple of weeks

Merge from HEAD the SMB signing patch that I developed a couple of weeks
ago.

This patch re-adds support for 'optional' SMB signing.  It also ensures that
we are much more careful about when we enable signing, particularly with
on-the-fly smb.conf reloads.

The client code will now attempt to use smb signing by default, and disable
it if the server doesn't correctly support it.

Andrew Bartlett
(This used to be commit e27b5cbe75d89ec839dafd52dd33101885a4c263)