Set Ctty in SysProcAttr (#75)

go.mod

diff --git a/go.mod b/go.mod
index 4a275a5..f73bcd6 100644
--- a/go.mod
+++ b/go.mod
@@ -1 +1,3 @@
 module github.com/kr/pty
+
+go 1.12

run.go

diff --git a/run.go b/run.go
index 56a26ad..959be26 100644
--- a/run.go
+++ b/run.go
@@ -47,6 +47,7 @@
 	}
 	c.SysProcAttr.Setctty = true
 	c.SysProcAttr.Setsid = true
+	c.SysProcAttr.Ctty = int(tty.Fd())
 	err = c.Start()
 	if err != nil {
 		pty.Close()