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Optionally treat EOF on stdin just like SIGTERM (#111)

* Optionally treat EOF on stdin just like SIGTERM

On Unix, Node.js allows us to gracefully kill a process. On Windows
this is more compex. You certainly cannot rely on the default `kill()`
function, which calls `TerminateProcess`.

There is a bunch of C/C++ extensions that in principle allow you to
attempt to gracefully shutdown a Windows process.

But, hey, here's a reality check. Node.js controls our stdin. Node.js
does IPC easy. Controlling uv_spawn flags and using the right not well maintained
C/C++ Node.js extension to kill a process is fragile.

So, treat EOF and any other error on stdin as equivalent to SIGTERM.

However, systemd.

The sane thing to do with systemd is `StandardInput=null`. With such
configuration, stdin immediately returns EOF.

Then, introduce the `OONI_STDIN_EOF_IMPLIES_SIGTERM` environment
variable. When it is `true`, this behaviour is enabled, e.g.:

```bash
export OONI_STDIN_EOF_IMPLIES_SIGTERM=true  # behaviour enabled
ooniprobe run
```

I want the default to be disabled because:

1. in the future we may find a better way to solve this problem and I
don't want the _default behaviour_ to change in such case

2. we know we need this knob for ooniprobe-desktop, and we will not
fail to provide it, so it won't suprise/damage us

3. a person trying to write a systemd unit for ooniprobe would be very
surprised to find out they need to disable this behaviour, if it was
enabled by default by this PR

Hence, I believe this design is consistent with designing for the
future and for trying to minimize surprises.

Also, why an environment variable and not a command line flag? Because:

1. we don't want such hypothetical flag to be available where it does not
make sense, e.g., for all subcommands but `run`

2. we don't want the ooni/probe-desktop app to write conditional
code because it needs to check the command we're using and th... (continued)

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