x/sys/unix: missing constants, errors for *xattr functions on linux.

[hirochachacha]

XATTR_CREATE and XATTR_REPLACE are missing.
"mkerror.sh" doesn't care about "sys/xattr.h".

http://man7.org/linux/man-pages/man2/setxattr.2.html

[gopherbot]

CL https://golang.org/cl/46690 mentions this issue.