runtime: block console ctrlhandler when the signal is handled

Fixes #41884

I can confirm this change fixes my issue.
I can't confirm that this doesn't break any and everything else.
I see that this code has been tweaked repeatedly, so I would really welcome guidance into further testing.

Change-Id: I1986dd0c2f30cfe10257f0d8c658988d6986f7a6
GitHub-Last-Rev: 92f02c96973e12f1472511bcf3c5ebb36c6b0440
GitHub-Pull-Request: golang/go#41886
Reviewed-on: https://go-review.googlesource.com/c/go/+/261057


Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Alex Brainman <alex.brainman@gmail.com>