SD: File open with O_WRITE would always append, even when O_APPEND is…

[SodaqMoja]

… off

Without this fix it is not possible to open an existing file and truncate
the contents.

[sandeepmistry]

Hi @SodaqMoja,

Thank you for submitting this pull request. Could you please resubmit to the following repository: https://github.com/arduino-libraries/SD

The libraries/SD folder is synced from this repo.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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