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/46690Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>