Upgrade MailScanner

[JohnMertz]

There are fairly light, but essential modifications made to the version of MailScanner shipped with the Jessie release.

It is necessary that we ship an updated version of MailScanner with Next, which means porting our modifications. I will start by simply porting the patches directly to a fork, but will then see if we can accomplish what we need with a CustomConfig module only. See MailWatch as an example of how to integrate a custom module.

As long as we can still write to the SpamCheck header, we should not need to modify the MailScanner code directly. If we can do so with a module, this should allow us to simply use the upstream version of MailScanner with a custom config only.

[JohnMertz]

I've gone through and re-applied all of the changes that we have in /opt/MailScanner on our up-to-date appliance to the files as they existed upstream in the version used in Current:

SpamTagger/MailScanner-v4@f6f8698

I'm now working on evaluating which patches need to be applied to the latest v5 version and which could be migrated to a custom module.

For lack of a better place to put this, note that SweepViruses.pm does not need to be updated since it already has support of ESET and adds upstream support to not retry on batch failure with the option Retry Batch Scanner = no. Note that ClamAV support is deprecated in favor of just using the existing clamd socket. I need to implement this in our config file.