This commit improves the interface_aix.go file, based on feedbacks about
CL 138724.

To retrieve MTU, ioctl is needed. It's implemented inside
internal/syscall/unix.

Change-Id: Ic583d26b93935a32a5f1eb5a2170b86e80a4a85e
Reviewed-on: https://go-review.googlesource.com/c/142157
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>