term: unify ioctl{Read,Write}Termios definition for aix, linux, solaris & zos

These all use TCGETS/TCSETS. Rather than having a term_unix_$GOOS.go
file for each platform, unify them in a single file.

Change-Id: Iefeeadf9cf53d8fa32b1b15b8169744a819565c8
Reviewed-on: https://go-review.googlesource.com/c/term/+/314231
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser

term_unix_linux.go

@@ -1,9 +1,9 @@
-// Copyright 2019 The Go Authors. All rights reserved.
+// Copyright 2021 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix
-// +build aix
+//go:build aix || linux || solaris || zos
+// +build aix linux solaris zos
 
 package term