Transaction file triggers don't respect priority

[dmnks]

While per-package file triggers can influence whether they're executed before or after standard scriptlets (if -P is greater than 100000, they'll be executed before, otherwise after standard scriptlets), their per-transaction counterparts cannot. This is because we execute these in a fixed order at the moment.

In a way, this is an RFE, but then, our documentation makes it seem as though -P works for every type of file trigger, so it might as well be considered a bug. The documentation error would be much easier to fix, however there's at least one use case out there that would benefit from the priorities actually working, hence this ticket.