Add support for out-of-order audit messages

I've made a custom `audit.Assembler` to handle auditd messages at the end of a log pipeline that doesn't guarantee ordered delivery. This seems like a generally useful thing, so would you accept some PRs to get this added in?

I think this would mean refactoring `libaudit.Reassembler` to an interface and implementing a `UnorderedReassembler` struct that emits a slice of messages when it gets a complete set. FWIW, our `NewUnorderedReassembler()` has the same args as `NewReassembler()` and is used basically the same way.

I promise to also commit a bunch of tests at the same time. :)