unix: add Setxattr constants

Add the XATTR_CREATE and XATTR_REPLACE constants, to be used in the
flags parameter of Setxattr/Lsetxattr. See
http://man7.org/linux/man-pages/man2/setxattr.2.html

Fixes golang/go#14454

Change-Id: I868ac019fe3be755e0f6f8bff734f843ba602ee9
Reviewed-on: https://go-review.googlesource.com/46690
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/unix/zerrors_linux_386.go b/unix/zerrors_linux_386.go
index e8d945e..a21c3ab 100644
--- a/unix/zerrors_linux_386.go
+++ b/unix/zerrors_linux_386.go
@@ -1788,6 +1788,8 @@
 	WORDSIZE                             = 0x20
 	WSTOPPED                             = 0x2
 	WUNTRACED                            = 0x2
+	XATTR_CREATE                         = 0x1
+	XATTR_REPLACE                        = 0x2
 	XCASE                                = 0x4
 	XTABS                                = 0x1800
 )