bpo-38858: Add init_interp_main() subfunction

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Fix new_interpreter() error handling: undo it all if status is an
exception.

https://bugs.python.org/issue38858

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Travis CI failed with ENV_CHANGED. It's unrelated to my change. See: https://bugs.python.org/issue38546

test_map_timeout (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 6.17s ok

test_max_workers_negative (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 0.16s ok

test_max_workers_too_large (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... skipped 'Windows-only process limit'

test_no_stale_references (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 0.18s ok

test_ressources_gced_in_workers (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 1.47s ok

test_shutdown_race_issue12456 (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 0.14s ok

test_submit (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 0.14s ok

test_submit_keyword (test.test_concurrent_futures.ProcessPoolForkProcessPoolExecutorTest) ... 0.14s
Warning -- reap_children() reaped child process 23508
ok