random(3): Abstract routines into _r versions on explicit state

[cem]

Summary

The existing APIs simply pass the implicit global state to the _r variants.

No functional change.

[kevans]

I had some concerns about providing *_r variants of these that match names from glibc but aren't compatible, but I see the ultimate plan is that these aren't intended to be exported from libc at all- this seems OK.

[cem]

In D23289#510876, @kevans wrote:

I had some concerns about providing *_r variants of these that match names from glibc but aren't compatible, but I see the ultimate plan is that these aren't intended to be exported from libc at all- this seems OK.

Hm, I didn't realize glibc had these routines. They picked different argument orderings and state struct name (and field names), but otherwise look pretty similar. Yeah, I didn't intend to export these routines from libc and don't see a great reason to.