Add support for fs.fdatasync()

[humphd]

https://nodejs.org/api/fs.html#fs_fs_fdatasync_fd_callback

[modeswitch]

Is this for compatibility reasons only? According to the docs it's possible for a transaction to report completion before data is flushed to physical storage. There's also no standardization (I think?) between implementations around this behavior. Firefox uses a non-standard API to allow client code to force flushing before completion.

[humphd]

Only for compat, yes, it will be essentially a no-op.