terminal/ssh: use ioctl wrappers from x/sys/unix
terminal/ssh: use ioctl wrappers from x/sys/unix

Use the ioctl wrapper functions from x/sys/unix instead of manually
re-implementing them.

Change-Id: I224de0c6ec7439dfd8c45c72071c947be8813d6a
Reviewed-on: https://go-review.googlesource.com/75991
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/term