This ioctl is used to implement ptsname on netbsd.

Change-Id: Ic87f1bf7d15c6fbef0c2226a06a4983a504c3f30
Reviewed-on: https://go-review.googlesource.com/c/148097
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>