Implement uv_fs_mkdtemp

I implemented `uv_fs_mkdtemp` for unices and created a test for that. Could someone confirm that I am on the right way or provide some suggestions before I start implementing `uv_fs_mkdtemp` for Windows? Specifically I'm in doubt whether `uv_fs_t->ptr` is the right place for storing the name of temp directory which was created by underlying `mkdtemp`.

For Windows I want to use `glibc`'s [implementation](http://fossies.org/dox/glibc-2.19/mkdtemp_8c_source.html) which uses [`__gen_tempname`](http://fossies.org/dox/glibc-2.19/sysdeps_2posix_2tempname_8c_source.html#l00229).

Should close #1315
@txdv sorry, I don't understand what you want to say in your last comment, but I want to point you to https://github.com/Hinidu/libuv/blob/mkdtemp/src/unix/fs.c#L91
This `PATH` as you can see is used in many functions and it does the same thing as my code here. Actually I written it explicit by @saghul's suggestion because `PATH` works with `path` argument but in case of `uv_fs_mkdtemp` argument is `template`.