Specific GetOSRandom for Linux/FreeBSD/OpenBSD

Summary: This is backport from PR9821 from core.

Test Plan:

make check

Reviewers: freetrader, sickpig, kyuupichan, CCulianu, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D488