bsd: init netbsd & openbsd userland

Adds a couple of useful NetBSD and OpenBSD derivations. Some of these
will be integrated into Nixpkgs later.

Noncomprehensive list:

- netbsd.getent
- netbsd.getconf
- netbsd.fts
- openbsd.mg
- netbsd.compat (can replace libbsd)

pkgs/os-specific/bsd/netbsd/fts-setup-hook.sh

@@ -0,0 +1,16 @@
+ftsLdflags() {
+    # The `depHostOffset` describes how the host platform of the dependencies
+    # are slid relative to the depending package. It is brought into scope of
+    # the environment hook defined as the role of the dependency being applied.
+    case $depHostOffset in
+        -1) local role='BUILD_' ;;
+        0)  local role='' ;;
+        1)  local role='TARGET_' ;;
+        *)  echo "cc-wrapper: Error: Cannot be used with $depHostOffset-offset deps" >2;
+            return 1 ;;
+    esac
+
+    export NIX_${role}LDFLAGS+=" -lfts"
+}
+
+addEnvHooks "$hostOffset" ftsLdflags