Implement fs.fsync()

[humphd]

It's not clear if we need this for anything other than compatibility with node.js (i.e,. it could be a no-op if necessary). However, we should add it for that at least.

See:

• http://nodejs.org/api/fs.html#fs_fs_fsync_fd_callback
• https://github.com/joyent/node/blob/master/lib/fs.js#L615
• http://pubs.opengroup.org/onlinepubs/009695399/functions/fsync.html

[humphd]

Fixed in #593