make latexmk continuous under windows more robust

This makes latexmk continuous work under Windows for every Windows Version from XP Home onward and for gVim and Vim under every Terminal.

Problems before: 

* Only worked under gVim together with cmd shell. 
* Only worked from Windows 7 on or newer.
* If latexmk running twice for the same main TeX file, then wrong PID used.

The details of the implementation are commented.