Parse the transaction problem strings into something marginally less useless

Ugh this is so gross
Remove the probfilter argument from TransactionSet.run

Of course it doesn't exist in rpm-4.4. We already filter things out on the
problems end, so maybe it doesn't matter? Haha, who knows.
Fix our expectations for the return value from rpmts_Run

The ancient version of rpmts_Run does not return an integer return code. Why
would it? Rekajigger everything to expect list-or-None instead.
Use str.split(sep, 1) instead of str.partition(sep)

str.partition(sep): New in version 2.5.