Calling directly into Win API to get current process exe

Calling directly into Win API to get current process exe

On Windows the MonoToolsLocator tries to determine which mono.exe
to use by looking at the current process. Due to problems with
AppDomains and shadow copying (which happens in System.Web) we
could not use this approach. Instead we now call GetModuleFileName
directly using p-invoke to get the exe file of the running process.
This is only used when running on Windows.