Add IoctlGetInt/IoctlSetInt, IoctlGetTermios/IoctlSetTermios and
IoctlGetWinsize/IoctlSetWinsize on OpenBSD. These are similar to the
already existing implementations on Linux, Darwin and Solaris.

Change-Id: I99e9434fa50338a0b5d825490513e9383539727d
Reviewed-on: https://go-review.googlesource.com/75791
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>