* bpo-1054041: Exit properly by a signal after a ^C.

An uncaught KeyboardInterrupt exception means the user pressed ^C and
our code did not handle it.  Programs that install SIGINT handlers are
supposed to reraise the SIGINT signal to the SIG_DFL handler in order
to exit in a manner that their calling process can detect that they
died due to a Ctrl-C.  https://www.cons.org/cracauer/sigint.html

After this change on POSIX systems

 while true; do python -c 'import time; time.sleep(23)'; done

can be stopped via a simple Ctrl-C instead of the shell infinitely
restarting a new python process.

What to do on Windows, or if anything needs to be done there has not
yet been determined.  That belongs in its own PR.

TODO(gpshead): A unittest for this behavior is still needed.

* Do the unhandled ^C check after pymain_free.

* Return STATUS_CONTROL_C_EXIT on Windows.

* Fix ifdef around unistd.h include.

* 📜🤖 Added by blurb_it.

* Add STATUS_CTRL_C_EXIT to the os module on Windows

* Add unittests.

* Don't send CTRL_C_EVENT in the Windows test.

It was causing CI systems to bail out of the entire test suite.

See https://dev.azure.com/Python/cpython/_build/results?buildId=37980
for example.

* Correct posix test (fail on macOS?) check.

* STATUS_CONTROL_C_EXIT must be unsigned.

* Improve the error message.

* test typo :)

* Skip if the bash version is too old.

...and rename the windows test to reflect what it does.

* min bash version is 4.4, detect no bash.

* restore a blank line i didn't mean to delete.

* PyErr_Occurred() before the Py_DECREF(co);

* Don't add os.STATUS_CONTROL_C_EXIT as a constant.

* Update the Windows test comment.

* Refactor common logic into a run_eval_code_obj fn.