runtime: always install new signal stack on NetBSD and DragonFly
On NetBSD and DragonFly a newly created thread inherits the signal stack
of the creating thread. That means that in a cgo program a C thread
created using pthread_create will get the signal stack of the creating
thread, most likely a Go thread. This will then lead to chaos if two
signals occur simultaneously.
We can't fix the general case. But we can fix the case of a C thread
that calls a Go function, by installing a new signal stack and then
dropping it when we return to C. That will break the case of a C thread
that calls sigaltstack and then calls Go, because we will drop the C
thread's alternate signal stack as we return from Go. Still, this is
the 1.5 behavior. And what else can we do?