Add support for the lgetxattr, llistxattr, lremovexattr and lsetxattr on
Linux, akin to the already existing getxattr, listxattr, removexattr and
setxattr syscalls.

These variants are identical to the already existing variants. except in
the case of a symbolic link where the extended attribute operation is
applied to the link itself rather than to the file that it refers to.
See:

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

Fixes golang/go#14116

Change-Id: I786953db8bf862566f54bdd745cc1feca5031be7
Reviewed-on: https://go-review.googlesource.com/44374Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>