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