[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 would do this on a separate file os/os_defs.h - similar to what currently happens with os_unix_defs.h and os_unix.h. - to keep platform specific defines away from internal os_* stuff.