This reverts the changes from https://golang.org/cl/33018: Instead
of writing the result of gofmt to a tmp file and then rename that
to the original (which doesn't preserve the original file's perm
bits, uid, gid, and possibly other properties because it is hard
to do in a platform-independent way - see #17869), use the original
code that simply overwrites the processed file if gofmt was able to
create a backup first. Upon success, the backup is removed, otherwise
it remains.

Fixes #17873.
For #8984.

Change-Id: Ifcf2bf1f84f730e6060f3517d63b45eb16215ae1
Reviewed-on: https://go-review.googlesource.com/33098
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>