Document and stablize functions from rustix::runtime.

[sunfishcode]

We should work towards moving functions currently in rustix::runtime out into the public API modules. This should involve thinking about their safety conditions, and figuring out what kind of public API works for the expected use cases. Also, rustix::runtime is not currently implemented for the libc backend, and it's desirable for rustix to expos the same features across its two backends, so that users (including indirect users) have the choice of whether to use the libc backend.

For example, rustix::runtime::sigaction exposes kernel-level signal registration function that bypasses libc. Libc implementations don't expect this to ever happen, so doing so may invoke undefined behavior. The precise conditions aren't documented, and could change with libc versions. As far as I can tell, the only guidance we could give for some functions would be "don't ever call this in a process which also contains a libc, or anything acting like a libc, including origin". And that's in addition to regular safety conditions, such as that the various raw pointers have to be used properly, and that don't SIGSEGV and SIGBUS shouldn't be ignored, because it could potentially bypass stack overflow guards, and all the safety considerations around signal handlers, and probably more.

In general, the main options for any given function in rustix::runtime include:

• Make it public in rustix, with sufficiently imposing documentation, and either add a libc backend implementation using libc::syscall (if it's not incompatible with libc), or add further documentation saying "using this will prevent your users from enabling rustix's libc backed; please consider what you do here" or so, or
• Have rustix expose a higher-level or more specialized API for it.
• Expose it through origin instead of rustix (eg. origin::signal::sigaction).

Which of these three is best for any given function depends on what kinds of use cases people will have. So for any function in rustix::runtime that anyone would like to see documented and stabilized, please comment about it here, and describe your use case!

[sunfishcode]

An example of pulling a function out of rustix::runtime is #1133. In making the function public, we're renaming it from the Linux-specific exit_group to the more descriptive immediately_exit, implementing it for the libc backend, and adding a sufficiently imposing documentation comment.

[sunfishcode]

I should also say, PRs are welcome but not necessary here. Just posting a comment here would be helpful!