problem with signal handling and parallel GC on linux

[ikod]

Issue https://issues.dlang.org/show_bug.cgi?id=20256

Conservative GC scan threads do not block signals when created, which can lead to unexpected behaviour if developer have to block signals for whole process.

This PR fix issue by blocking all signals in conservative GC scanThreads.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20256	regression	problem with signal handling and parallel GC on linux

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + druntime#2813"

[rainers]

It's Posix, not posix.

[rainers]

Apart from nits, LGTM.

Please squash commits.

[rainers]

I guess you mean scanBackground here.

[rainers]

code style nit: for (int i = 0; i < 10000; i++)

[wilzbach]

Bonus for foreach (i; 0 .. 10000)

[rainers]

nit: if (child_pid == 0)

[ikod]

Thanks!