This corresponds to https://github.com/msys2/msys2-runtime/pull/239 and
integrates one minor change of behavior where `remove_proc()` now _also_
tries `CancelSynchronousIo()` before using the big `terminate_thread()`
hammer.
Probably not a very noticeable change of behavior. But still. Better
stay aligned with Cygwin!
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>