Restore support for older Go versions

When compiled with go older than 1.12 creack/pty will not include a fix
for blocking Read() and will be prone to data races - but at least it will work

For more information see issues: #88 #114 #156 #162

io_test.go

@@ -1,3 +1,6 @@
+//go:build go1.12
+// +build go1.12
+
 package pty
 
 import (

ioctl.go

@@ -1,7 +1,6 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && go1.12
+// +build !windows,go1.12
 
-//
 package pty
 
 import "os"

ioctl_legacy.go

@@ -0,0 +1,10 @@
+//go:build !windows && !go1.12
+// +build !windows,!go1.12
+
+package pty
+
+import "os"
+
+func ioctl(f *os.File, cmd, ptr uintptr) error {
+	return ioctl_inner(f.Fd(), cmd, ptr) // fall back to blocking io (old behavior)
+}