runtime: revert CL 18835; don't install new signal stack unconditionally on dragonfly
This change reverts CL 18835 which is a workaroud for older DragonFly
BSD kernels, and fixes #14051, #14052 and #14067 in a more general way
the same as other platforms except NetBSD.
This change also bumps the minimum required version of DragonFly BSD
kernel to 4.4.4.
Fixes #16329.
Change-Id: I0b44b6afa675f5ed9523914226bd9ec4809ba5ae
Reviewed-on: https://go-review.googlesource.com/29491 Reviewed-by: Ian Lance Taylor <iant@golang.org>