Ignored reports whether sig is currently ignored.

This implementation only works applies on Unix systems for now.  However, at
the moment that is also the case for Ignore() and several other signal
interaction methods, so that seems fair.

Fixes #22497

Change-Id: I7c1b1a5e12373ca5da44709500ff5acedc6f1316
Reviewed-on: https://go-review.googlesource.com/108376
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>