Convert file trigger scripts to lua

At least the %filetriggerpostun script can be invoked hundreds of
times during an upgrade, so it makes sense to optimize it a bit.

assert(exec(...)) is used because of https://bugzilla.redhat.com/show_bug.cgi?id=1094072.

Add -P (--priority) to have %filetriggerpostun run as early as
possible (before any reload/stop actions), and %transfiletriggerin as
late as possible (after any enable/disable/preset actions).