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>

util_bsd.go

diff --git a/util_bsd.go b/util_bsd.go
index 9c1ffd1..cb23a59 100644
--- a/util_bsd.go
+++ b/util_bsd.go
@@ -6,7 +6,7 @@
 
 package terminal
 
-import "syscall"
+import "golang.org/x/sys/unix"
 
-const ioctlReadTermios = syscall.TIOCGETA
-const ioctlWriteTermios = syscall.TIOCSETA
+const ioctlReadTermios = unix.TIOCGETA
+const ioctlWriteTermios = unix.TIOCSETA

util_linux.go

diff --git a/util_linux.go b/util_linux.go
index 5883b22..5fadfe8 100644
--- a/util_linux.go
+++ b/util_linux.go
@@ -4,8 +4,7 @@
 
 package terminal
 
-// These constants are declared here, rather than importing
-// them from the syscall package as some syscall packages, even
-// on linux, for example gccgo, do not declare them.
-const ioctlReadTermios = 0x5401  // syscall.TCGETS
-const ioctlWriteTermios = 0x5402 // syscall.TCSETS
+import "golang.org/x/sys/unix"
+
+const ioctlReadTermios = unix.TCGETS
+const ioctlWriteTermios = unix.TCSETS