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
I apologize. It was my mistake. I hadn't much experience with copyright law.
I found slightly different OpenBSD version here http://openbsd.cs.toronto.edu/cgi-bin/cvsweb/src/lib/libc/stdio/mktemp.c It seems like their license permits copying and modification.
I can try change current implementation according to this version.
Sorry again, I hope I didn't create a lot of problems.