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

Change-Id: I023214f3b65e3b7d620fa7e1ce12f1c35a765cd9
Reviewed-on: https://go-review.googlesource.com/c/145817
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>