The `invoke_default_handler` function

[vorner]

The current documentation states how one could emulate the individual default signal handlers. This works, but is a bit unergonomic.

The idea is to provide a wrapper function that takes a signal number, looks up what its default handler would be and invokes emulation of that one.

❓ We don't necessarily know about all possible signals on the given platform (some platforms have a bit more of them, someone might even have custom-patched kernel to add something more). What should the function do with these? Ignore? Return an error?