* temp regrtest save_env patch
* Refactor signal_handlers from Option to OnceCell
- Change `signal_handlers` type from `Option<Box<...>>` to
`OnceCell<Box<...>>` so fork children can lazily initialize it
- Add `VirtualMachine::is_main_thread()` using runtime thread ID
comparison instead of structural `signal_handlers.is_none()` check
- Initialize signal handlers in `py_os_after_fork_child()` via
`get_or_init()` for workers that fork
- Use `get_or_init()` in `signal()` and `getsignal()` functions
- Remove `set_wakeup_fd(-1)` special-case workaround (d6d0303)
- Extract `signal::new_signal_handlers()` to deduplicate init expr
- Allow `getsignal()` from any thread (matches CPython behavior)
- Fix `set_wakeup_fd` error message to name the correct function
There seems to have been a bug in the libregrtest code which unloaded
modules between tests. The previous state was calculated using
`sys.modules.keys()`, which is actually a mutable object that is updated
as the underlying `sys.modules` is updated. The result was that modules
were not unloaded between tests, which is the root cause for
`test_unittest` failing when run after `test_import` and
`test_importlib`.
This code is copied from 3.12. Ideally all of `libregrtest` should
probably be updated as it seems wildly out of date, but that's a lot
more work.
