Gather additional entropy from the environment

Summary:

This based on code by Gregory Maxwell.

Partial backport of core PR17270:
https://github.com/bitcoin/bitcoin/pull/17270/commits/2554c1b81bb8c40e1989025c6f18e7935720b156

Depends on D6161.

Test Plan:

ninja all check-all

Run the Gitian builds (will run symbol-check).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6162