Bug 1531030: Use MOZ_DISABLE_POISON_IO_INTERPOSER to disable PoisonIOInterposer when it is unsafe to initialize; r=erahm

If we are running a background thread in the launcher process to log failures,
then allowing the main thread to proceed with monkeypatching system calls is a
Bad Idea. This patch gives us an environment variable that, when set, indicates
that it is unsafe for PoisonIOInterposer to run.

This scenario is an uncommon one, but one that we must account for nonetheless.

Differential Revision: https://phabricator.services.mozilla.com/D21607

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extra : moz-landing-system : lando