cmd/gofmt: try to write original data on rewrite failure

When gofmt needs to rewrite a file, it first copies it into a backup.
If the rewrite fails, it used to rename the backup to the original.
However, if for some reason the file is owned by some other user,
and if the rewrite fails because gofmt doesn't have permission to
write to the file, then renaming the backup file will change
the file owner. This CL changes gofmt so that if it fails to rewrite
a file, it tries to write the original contents. If writing the original
content fails, it reports the problem to the user referring to the
backup file, rather than trying a rename.

Also create the backup file with the correct permissions,
to avoid a tiny gap when some process might get write access to the
file contents that it shouldn't have. (This tiny gap only applies to
files that are not formatted correctly, and have read-only permission,
and are in a directory with write permission.)

Fixes #60225

Change-Id: Ic16dd0c85cf416d6b2345e0650d5e64413360847
Reviewed-on: https://go-review.googlesource.com/c/go/+/495316
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

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