On Windows, terminate processes by handle, not pid

On Windows, terminate processes by handle, not pid

When we create a process yourself with CreateProcess, we get a
handle to the process in the PROCESS_INFO output structure.  But
instead of using that handle, we were manually looking up a _new_
handle based on the process ID, which is a poor idea, since the
process ID might refer to a new process later on, but the handle
can't.