GH196: Fix terminateJob

[Mistuke]

Hi,

terminateJob is being called from within terminateProcess which already has a lock on the MVar.
This add a new function terminateJobUnsafe which does not take an MVar and so is not thread-safe but can be called from terminateProcess safely.

Fixes #196

[snoyberg]

The AppVeyor tests failed on this build.

Side note: I think I'll move CI over to Github Actions instead of the Travis/AppVeyor split, just an FYI.

[Mistuke]

The AppVeyor tests failed on this build.

That test is somewhat broken which is why it sometimes passes and sometimes fails. See https://sourceforge.net/p/mingw/mailman/mingw-users/thread/CAO2ddnbqqTwABiLSpyDN-ynyQR%3D6fRt5Mt94gszkgQoJF-atOA%40mail.gmail.com/#msg30566147

In general there's no guarantee that the PID inside a Cygwin process is the same as the PID in Windows. Cygwin is trying to emulate a posix environment in user space. It will try to have the PIDs match but this may not always succeed which is why this test is flaky.

In any case, I've updated the test to read the winpid from procfs instead.

[Mistuke]

@snoyberg alright, the test is fixed to work properly on Windows now.

[snoyberg]

Looks great. One more request: could you add a changelog entry explaining the change?

[Mistuke]

Done.

[snoyberg]

Thanks!