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

src/libbsd.map

@@ -160,3 +160,11 @@ LIBBSD_0.9 {
     strsvisx;
     svis;
 } LIBBSD_0.8;
+
+LIBBSD_0.9.1 {
+    /* The strnvis() and strnunvis() symbols changed prototype to match
+     * the NetBSD implementation. Provided as versioned nodes in 0.9.1, and
+     * exposed here explicitly so that we can redirect at compile-time. */
+    strnvis_netbsd;
+    strnunvis_netbsd;
+} LIBBSD_0.9;