unix: export Ifreq and add IoctlIfreq function

This CL expands upon CL 340369 by exporting the Ifreq type along with methods
for setting and getting data to/from the ifreq union in a type-safe way.
ifreqData remains unexported as we can keep adding helpers similar to the
IoctlGetEthtoolDrvinfo to expose those operations in a less error-prone way.

A test is also added to verify interface index data using IoctlIfreq against
the modern rtnetlink API used by the standard library.

Change-Id: Ic6980cbcd3792cc341cd614061cce32fa1f851e7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/340370
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_internal_linux_test.go

@@ -14,14 +14,6 @@ import (
 	"unsafe"
 )
 
-func Test_ifreqSize(t *testing.T) {
-	// Ensure ifreq (generated) and ifreqData (hand-written due to
-	// unsafe.Pointer field) are identical in size.
-	if want, got := unsafe.Sizeof(ifreq{}), unsafe.Sizeof(ifreqData{}); want != got {
-		t.Fatalf("unexpected ifreq size: got: %d, want: %d", got, want)
-	}
-}
-
 func makeProto(proto int) *int {
 	return &proto
 }