sftp-server: Add nanoseconds support

[Opty]

On Sat, Sep 4, 2021 at 8:40 AM Ron Frederick <ronf at timeheart.net> wrote:
> There is a version of the SFTP protocol defined which supports sub-second times, but it’s not widely implemented. [...], whereas most SFTP implementations (including OpenSSH last time I checked) only implement version 3.
>
> It looks like the SUBSECOND_TIMES flag was added in version -04 of this draft (https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-04), which sets an SFTP version number of 4. Section 10 of this draft summarizes the proposed differences between versions 3 and 4:
>
> [...]
>
> It’s not legal to change the meaning of the SFTP file attribute flags that specify what attributes are present without changing the SFTP version number, so implementing sub-second files by itself would not be straightforward. You’d have to implement the rest of the changes proposed here as well, and it would only be useful if both the SFTP client and SFTP server had this new support and were able to negotiate the higher version number.

Maybe the time for protocol upgrade has come? O:-) I see some more
benefits than just nanoseconds.

Regards,
Opty