Currently Linux' fchmodat(2) syscall implementation doesn't support the
flags parameter (though it might in future versions [1]). Fchmodat in
x/sys/unix takes the parameter and (wrongly) passes it on to the syscall
which will ignore it.

According to the POSIX.1-2008 manual page [2], AT_SYMLINK_NOFOLLOW is
the only valid value for the flags parameter and EOPNOTSUPP should be
returned in case changing the mode of a symbolic link is not supported
by the underlying system.  EINVAL should be returned for any other value
of the flags parameter.

  [1] https://patchwork.kernel.org/patch/9596301/
  [2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html

Change the Fchmodat implementation accordingly and also add the
corresponding test.

Fixes golang/go#20130

Change-Id: I62e677e6674d3702eaf388af0ac3d7e623a35c24
Reviewed-on: https://go-review.googlesource.com/46474Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>