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

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

No functional change.

Note that these routines are not exported from libc and are not intended to be
exported. If someone wished to export them from libc (which I would
discourage), they should first be modified to match the inconsistent parameter
type / order of the glibc public interfaces of the same names.

I know Ravi will ask, so: the eventual goal of this series is to replace
rand(3) with the implementation from random(3) (D23290). However, I'd like to
wait a bit longer on that one to see if more feedback emerges.

Reviewed by: kevans, markm
Differential Revision: https://reviews.freebsd.org/D23289