On NetBSD and DragonFly a newly created thread inherits the signal stack
of the creating thread.  This breaks horribly if both threads get a
signal at the same time.  Fix this by dropping the signal stack in the
newly created thread.  The right signal stack will then get installed
later.

Note that cgo code that calls pthread_create will have the wrong,
duplicated, signal stack in the newly created thread.  I don't see any
way to fix that in Go.  People using cgo to call pthread_create will
have to be aware of the problem.

Fixes #13945.
Fixes #13947.

Change-Id: I0c7bd2cdf9ada575d57182ca5e9523060de34931
Reviewed-on: https://go-review.googlesource.com/18814
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>