Fix strnvis() and strnunvis() NetBSD ABI break

Fix strnvis() and strnunvis() NetBSD ABI break

The NetBSD implementations have different prototypes to the ones coming
from OpenBSD, which will break builds, and have caused segfaults at
run-time. We provide now both interfaces with different prototypes as
different version nodes allow selecting them at compile-time, defaulting
for now to the OpenBSD one to avoid build-time breakage, while emitting
a compile-time warning. Later on, in 0.10.0, we will be switching the
compile-time default to the NetBSD version.

Ref: http://gnats.netbsd.org/44977
Fixes: https://bugs.debian.org/899282