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
IIRC, NTFS can, so yes.
On Jul 23, 2014 9:43 AM, "Pavel Platto" <notifications@github.com> wrote:

> In src/win/fs.c:
>
> > @@ -721,6 +721,80 @@ void fs__mkdir(uv_fs_t* req) {
> >  }
> >
> >
> > +void fs__mkdtemp(uv_fs_t* req) {
> > +  static const WCHAR letters[] =
> > +    L"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
>
> Does it make sense to use both upper and lower case? AFAIK most Windows
> file systems don't distinguish them in file names.
>
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> <https://github.com/joyent/libuv/pull/1368/files#r15274807>.
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