[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 :-)
Yes, I'll do it ASAP. As we discussed earlier I think entire ```os.h``` should be splitted in the future.