unix: add more illumos ioctl wrappers

Expose Strioctl and Lifreq structs
Add helpers for working with them
Add additional wrapper functions

This work is in support of wireGuard/wireguard-go#39

Change-Id: I7a4d919f986ec977e2cd393eaf237d8c43bbc1cb
Reviewed-on: https://go-review.googlesource.com/c/sys/+/302831
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Ian Lance Taylor <iant@golang.org>

unix/syscall_illumos_test.go

@@ -0,0 +1,26 @@
+// Copyright 2021 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.
+
+//go:build illumos
+// +build illumos
+
+package unix_test
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestLifreqSetName(t *testing.T) {
+	var l unix.Lifreq
+	err := l.SetName("12345678901234356789012345678901234567890")
+	if err == nil {
+		t.Fatal(`Lifreq.SetName should reject names that are too long`)
+	}
+	err = l.SetName("tun0")
+	if err != nil {
+		t.Errorf(`Lifreq.SetName("tun0") failed: %v`, err)
+	}
+}