Add Plan9 support

[rminnich]

[creack]

Thank you for the PR!

Something is off with the build directive, can you take a look?

[MDr164]

Something is off with the build directive, can you take a look?

@rminnich your build tags are slightly off, I attached a patch that should fix them:

From 192e9680a44820c838964baa4f51bc82917b9307 Mon Sep 17 00:00:00 2001
From: Marvin Drees <[email protected]>
Date: Tue, 4 Mar 2025 18:20:15 +0100
Subject: [PATCH] Fix build tags for Plan9

Signed-off-by: Marvin Drees <[email protected]>
---
 ioctl_inner.go  | 2 +-
 start.go        | 2 +-
 winsize_unix.go | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/ioctl_inner.go b/ioctl_inner.go
index 0ded8bc..e8ec0ea 100644
--- a/ioctl_inner.go
+++ b/ioctl_inner.go
@@ -1,5 +1,5 @@
 //go:build !windows && !solaris && !aix && !plan9
-// +build !windows,!solaris,!aix
+// +build !windows,!solaris,!aix,!plan9

 package pty

diff --git a/start.go b/start.go
index 781bb68..213f7b3 100644
--- a/start.go
+++ b/start.go
@@ -1,5 +1,5 @@
 //go:build !windows && !plan9
-// +build !windows !plan9
+// +build !windows,!plan9

 package pty

diff --git a/winsize_unix.go b/winsize_unix.go
index 8fe007b..6eca932 100644
--- a/winsize_unix.go
+++ b/winsize_unix.go
@@ -1,5 +1,5 @@
 //go:build !windows && !plan9
-// +build !windows !plan9
+// +build !windows,!plan9

 package pty

--
2.47.1