In order to prevent tasks from running in a non-main thread,
wait for the previous task inside _try_send_to_primary_thread,
then schedule the next task. Add a main_thread_only execmodel
to distinguish this new behavior from the existing thread
execmodel, since users of the thread execmodel expect that
tasks can run in multiple threads concurrently. If concurrent
remote_exec requests are submitted for the main_thread_only
execmodel, then the channel will raise a RemoteError with
this message:

    concurrent remote_exec would cause deadlock for main_thread_only execmodel

In order for main_thread_only users to avoid this error,
remote_exec callers must use the returned channel to wait
for a task to complete before they call remote_exec again,
as demonstrated in test_assert_main_thread_only. Testing of
main_thread_only with pytest-xdist has shown that this behavior
is compatible with the existing pytest-xdist usage because it
never calls remote_exec more than once per gateway.

Fix init_popen_io to leave behind a sane state (0 and 1 file
descriptors open), in order to prevent "Bad file descriptor"
errors. Also fix test_stdouterrin_setnull to restore stdin
state, while relying on capfd to do this for stdout and stderr.

The "Bad file descriptor" error doesn't trigger reliably
because it's triggered by garbage collection of the Popen2IO
instance returned from init_popen_io as shown in this job:

https://github.com/pytest-dev/execnet/actions/runs/7955155978/job/21716386705?pr=243

=================================== FAILURES ===================================
__________________ test_stdouterrin_setnull[main_thread_only] __________________

execmodel = <ExecModel 'main_thread_only'>
capfd = <_pytest.capture.CaptureFixture object at 0x7fba8c333990>

    @pytest.mark.skipif("not hasattr(os, 'dup')")
    def test_stdouterrin_setnull(execmodel, capfd):
        gateway_base.init_popen_io(execmodel)
        os.write(1, b"hello")
>       os.read(0, 1)
E       OSError: [Errno 9] Bad file descriptor

testing/test_basics.py:254: OSError

…task_in_try_send_to_primary_thread