Add Fchownat, Linkat, Mkdirat, Mkfifoat, Mknodat, Openat, Readlinkat,
Renameat, Symlinkat and Unlinkat.

Change-Id: Id498b2f1a99f281599bc226a0c50a13172e36e4f
Reviewed-on: https://go-review.googlesource.com/c/145818
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>