fs: uv_fs_{open,read}_dir for unix

Tested on Linux, MacOS X and SmartOS.

Fixes #1430
@splinterofchaos I have updated my changes to use a newly defined function named `uv__fs_direntry_size` and [implemented as recommended](https://github.com/misterdjules/libuv/blob/fix-issue-1430/src/unix/fs.c#L151-L215) in the document mentioned above. Thanks again!