[FnApiRunner]multi-process runner does not terminate cleanly upon receiving SIGINT

The multi-process runner does not handle SIGINT gracefully. To reproduce, run wordcount.py using the "Run with multiprocessing mode" instructions from the first comment in BEAM-3645 (in Python 3).

Expected: wordcount terminates gracefully when Ctrl-C is pressed during pipeline execution (similarly to default direct runner)
Actual: wordcount hangs forever after printing the following once per worker:

```

Exception in thread run_worker:
Traceback (most recent call last):
File "/usr/lib/python3.6/threading.py",
line 916, in _bootstrap_inner
self.run()
File "/usr/lib/python3.6/threading.py", line 864, in
run
self._target(*self._args, **self._kwargs)
File "/usr/local/google/home/yifanmai/venv/wordcount/lib/python3.6/site-packages/apache_beam/runners/portability/local_job_service.py",
line 216, in run
'Worker subprocess exited with return code %s' % p.returncode)
RuntimeError:
Worker subprocess exited with return code 1

```

Imported from Jira [BEAM-8149](https://issues.apache.org/jira/browse/BEAM-8149). Original Jira may contain additional context.
Reported by: hannahjiang.

Reproduce the hang with wordcount.py using the multiprocessing instructions from BEAM-3645, then inspect local_job_service.py around the run_worker traceback and worker subprocess handling. Done means Ctrl-C during pipeline execution exits cleanly without hanging, matching the direct runner behavior.

Written by the indexing model from the issue text.