Add Plan9 support

ioctl_inner.go

@@ -1,4 +1,4 @@
-//go:build !windows && !solaris && !aix
+//go:build !windows && !solaris && !aix && !plan9
 // +build !windows,!solaris,!aix
 
 package pty

ioctl_unsupported.go

@@ -1,5 +1,5 @@
-//go:build aix
-// +build aix
+//go:build aix || plan9
+// +build aix plan9
 
 package pty
 

start.go

@@ -1,5 +1,5 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && !plan9
+// +build !windows !plan9
 
 package pty
 

start_plan9.go

@@ -0,0 +1,11 @@
+package pty
+
+import (
+	"os"
+	"os/exec"
+)
+
+// StartWithSize can be supported after a fashion on Plan 9, later.
+func StartWithSize(cmd *exec.Cmd, ws *Winsize) (*os.File, error) {
+	return nil, ErrUnsupported
+}

winsize_unix.go

@@ -1,5 +1,5 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && !plan9
+// +build !windows !plan9
 
 package pty
 

winsize_unsupported.go

@@ -1,5 +1,5 @@
-//go:build windows
-// +build windows
+//go:build windows || plan9
+// +build windows plan9
 
 package pty