Chmod toggles the FILE_ATTRIBUTES_READONLY flag depending on the
permission bits. That's a bit odd but I guess some compromises were made
at some point and this is what was chosen to map to a Unix concept that
Windows doesn't really have in the same way. That's fine. However, the
logic used in Chmod was forgotten from Open, which then manifested
itself in various places, most recently, go modules' read-only behavior.

This corresonds with the syscall CL 202439.

Updates golang/go#35033

Change-Id: Id8e74c5205057a74a35eda213516780b79a2aed2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/202440
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

In the fix for golang/go#35033 Go started creating
read-only files on Windows when no read permission bit was requested in the
OpenFile call. This was not documented in the release notes (but see
https://go-review.googlesource.com/c/go/+/223957/).

Deal with the immediate issue by adding owner-read and owner-write permissions
in our WriteFile which creates the file.

Also add a workaround for the fact that previous version of Fleetspeak have left
behind read-only files - simply clear the read-only attribute if the file
already exists.

For #35033

Change-Id: Ie15353322d5cfe7320199103ad9543fb89a842ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/223957
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

…ermissions

For #35033
For #36878

Change-Id: Ie15353322d5cfe7320199103ad9543fb89a842ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/223957
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit e39de05)
Reviewed-on: https://go-review.googlesource.com/c/go/+/223962
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>