Main thread recognition broken when embedding

[smurfix]

Trying to use Trio in a Python installation that already has installed a C level SIGINT handler fails.

        File "/usr/lib/python3/dist-packages/trio/_util.py", line 79, in is_main_thread
          signal.signal(signal.SIGINT, signal.getsignal(signal.SIGINT))
        File "/usr/lib/python3.11/signal.py", line 56, in signal
          handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      TypeError: signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object