s3-winbindd: Require SMB signing by default to disrupt MITM attacks with our DC

s3-winbindd: Require SMB signing by default to disrupt MITM attacks with our DC

This makes it much harder to impersonate the DC, but allows this to be
turned off or returned to IF_REQUIRED with a simple change to the
'client signing' smb.conf parameter.

Andrew Bartlett

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Sun Sep 28 06:25:55 CEST 2014 on sn-devel-104