[RFC] Implement mkdtemp for Windows and refactor vim_tempname etc.

Should close #812.
Not much was done yet: just ported ```mkdtemp``` from glibc.
I hope that I'll can share my thoughts on that issue and continue to work on it tomorrow :-)
I have in mind two reasons to not doing it this way:
- It is a temporarily solution and should be removed in the future.
- Including "general" headers in some "main" file is similiar to the current situation with ```vim.h``` and is against "include what you use" goal.