ssh/terminal: use termios ioctl read/write constants from x/sys/unix

Use the TCGETS/TCSETS and TIOCGETA/TIOCSETA definitions from x/sys/unix
instead of manually declaring them or using the corresponding
definitions from syscall.

Change-Id: I37c2c8124d251eb47467b4184a7cc39781775f11
Reviewed-on: https://go-review.googlesource.com/51690
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed