random-util: use RDRAND for randomness if the kernel doesn't want to give us any

random-util: use RDRAND for randomness if the kernel doesn't want to give us any

Pretty much all intel cpus have had RDRAND in a long time. While
CPU-internal RNG are widely not trusted, for seeding hash tables it's
perfectly OK to use: we don't high quality entropy in that case, hence
let's use it.

This is only hooked up with 'high_quality_required' is false. If we
require high quality entropy the kernel is the only source we should
use.