[PATCH nft 1/3] meta: add random and probability match

[Florian Westphal <fw@strlen.de>]

Hi.

This is yet another attempt at the random/probability matching.

First patch adds a simple 'meta random' expression.

Second patch adds a 'probability' mode which internally maps to
the random one and takes care of scaling the '0.x' probability
to the needed integer.

Third patch is the reverse translation, so that users that
input

meta probability 0.001

will see the same on list instead of

meta random <= 4294967

Patch 2 and 3 use meta statement to add this probability mode.

Let me know, if needed I can also only push patch #1.

I also have a patch for meta.t to add tests for this.