plan9: reduce the set of architecture-dependent files on Plan 9

From main repo: https://go-review.googlesource.com/#/c/2167

Change-Id: I2043458a432ccd2aa8b206adc107ac7b215e355f
Reviewed-on: https://go-review.googlesource.com/8720
Reviewed-by: Rob Pike <r@golang.org>

plan9/errors_plan9.go

@@ -0,0 +1,50 @@
+// Copyright 2011 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.
+
+package plan9
+
+import "syscall"
+
+// Constants
+const (
+	// Invented values to support what package os expects.
+	O_CREAT    = 0x02000
+	O_APPEND   = 0x00400
+	O_NOCTTY   = 0x00000
+	O_NONBLOCK = 0x00000
+	O_SYNC     = 0x00000
+	O_ASYNC    = 0x00000
+
+	S_IFMT   = 0x1f000
+	S_IFIFO  = 0x1000
+	S_IFCHR  = 0x2000
+	S_IFDIR  = 0x4000
+	S_IFBLK  = 0x6000
+	S_IFREG  = 0x8000
+	S_IFLNK  = 0xa000
+	S_IFSOCK = 0xc000
+)
+
+// Errors
+var (
+	EINVAL       = syscall.NewError("bad arg in system call")
+	ENOTDIR      = syscall.NewError("not a directory")
+	EISDIR       = syscall.NewError("file is a directory")
+	ENOENT       = syscall.NewError("file does not exist")
+	EEXIST       = syscall.NewError("file already exists")
+	EMFILE       = syscall.NewError("no free file descriptors")
+	EIO          = syscall.NewError("i/o error")
+	ENAMETOOLONG = syscall.NewError("file name too long")
+	EINTR        = syscall.NewError("interrupted")
+	EPERM        = syscall.NewError("permission denied")
+	EBUSY        = syscall.NewError("no free devices")
+	ETIMEDOUT    = syscall.NewError("connection timed out")
+	EPLAN9       = syscall.NewError("not supported by plan 9")
+
+	// The following errors do not correspond to any
+	// Plan 9 system messages. Invented to support
+	// what package os and others expect.
+	EACCES       = syscall.NewError("access permission denied")
+	EAFNOSUPPORT = syscall.NewError("address family not supported by protocol")
+)