Renameat2 is Linux-specific and allows to pass an additional flags
argument to the renameat syscall. Also add the corresponding RENAME_*
flag mask values.

Change-Id: Ib4baa46fdd0b07ccd0988ac624862ce69d5c3ad5
Reviewed-on: https://go-review.googlesource.com/127763
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>