Make forknotify.py more robust

Summary:
Backport from https://github.com/bitcoin/bitcoin/commit/a4fd89fddba49c56e77f131fe95feffa8d7cf6ed

Fixes T144 / T145

(a4fd89f backported here)
(1f3d78b already backported at D972)

Test Plan: ./test/functional/test_runner.py forknotify

Reviewers: deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Maniphest Tasks: T144

Differential Revision: https://reviews.bitcoinabc.org/D979