unix: add Uvmexp and SysctlUvmexp for NetBSD

NetBSD has both struct uvmexp (used internally) and struct uvmexp_sysctl
(preferred for interfacing from user code). Expose the latter as type
Uvmexp with the same interface as on OpenBSD.

I am not sure if it makes sense to even have a name parameter for
SysctlUvmexp, since the only valid value can be "vm.uvmext2".

Change-Id: I40a743536c28e3fb7a54253768db35b1629fabfe
Reviewed-on: https://go-review.googlesource.com/c/sys/+/460156
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/sysctl_netbsd_test.go

@@ -0,0 +1,15 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "testing"
+
+func TestSysctlUvmexp(t *testing.T) {
+	uvm, err := SysctlUvmexp("vm.uvmexp2")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("%#v", uvm)
+}