Kernel/LibC: Implement rfork()

[Kusekushi]

This should close #1483 unless other flags are deemed neccessary/useful.

For flags refer to https://man.freebsd.org/cgi/man.cgi?query=rfork&sektion=2
This implements

• RFPROC (not really, we just check if this is present to be somewhat BSD compatible)
• RFNOWAIT
• RFFDG
• RFCFDG

[spholz]

I don't think we need this syscall. Especially since we have a posix_spawn syscall.
I'm generally not really a fan of clone()/rfork().

Is there any reason why you adding this syscall? We generally don't add such features without anything using it.

[Hendiadyoin1]

apart from spholz' comment, this seems to contain a small bit of refactoring, which -as separate commits-
may be nice to have on their own, and will make this generally more reviewable when separated

[stale]

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