dkim_sig_ignore() can be called from within the prescreen
callback, if defined.
Signature-specific handle, passed to the prescreen callback
Copyright (c) 2007 Sendmail, Inc. and its suppliers.
All rights reserved.
Copyright (c) 2009, 2010, The Trusted Domain Project. All rights reserved.
By using this file, you agree to the terms and conditions set
forth in the respective licenses.