fs: Fix default params for fs.write(Sync)

Add support for fs.write(fd, buffer, cb) and fs.write(fd, buffer, offset, cb)
as documented at
https://nodejs.org/api/fs.html#fs_fs_write_fd_data_position_encoding_callback
and equivalently for fs.writeSync

Update docs and code comments to reflect the implementation.

PR-URL: https://github.com/nodejs/node/pull/7856


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>