fs: uv_fs_{open,read}_dir for unix

Tested on Linux, MacOS X and SmartOS.

Fixes #1430
It's actually about a year older, I added it in commit d3f60da6.  :-)

It's been a while but I think I may have added it as a workaround for a Cygwin header bug (which we still supported at that time.)  4096 is the PATH_MAX on Linux, by the way.