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

Change-Id: I5a5a7ef69f02e01a4216779df5c927f241e5cec0
Reviewed-on: https://go-review.googlesource.com/76170Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>