Before this fix, opening a file with `O_CREATE|O_RDONLY` caused an IO error to
be returned when using `--vfs-cache-mode off` or `--vfs-cache-mode writes`.
This was because the file was opened with read intent, but the `O_CREATE`
implies write intent to create the file even though the file is opened
`O_RDONLY`.
This fix sets write intent for the file if `O_CREATE` is passed in which fixes
the problem for all the VFS cache modes.
It also extends the exhaustive open flags testing to `--vfs-cache-mode writes`
as well as `--vfs-cache-mode full` which would have caught this problem.
See: https://forum.rclone.org/t/i-o-error-trashing-file-on-sftp-mount/34317/
Nick Craig-Wood
46b080c092