Authorize.net recurring back-office membership transactions silent failure

When creating backoffice recurring membership payments for Authorize.net in 4.4 (& probably earlier based on DB) the contact ID is not passed to Authorize.net. The transaction completes successfully but CiviCRM doesn't not update as the IPN is set to abort if x_cust_id is not provided.

There are 2 parts to this
#1) fix set up of these transactions
#2) get CiviCRM to stop rejecting existing ones that don't have x_cust_id in the IPN. This id is not required for matching purposes - it is one of 3 ids used to verify a true match - so we should be able to reduce to just 2 -

Unfortunately this is a data-loss

I have a PR for #1 at this stage (simply opening the file in phpstorm makes the cause of the problem very apparent)