feat: Use type parameters to allow {get,set}regset to use different register set structs

[hack3ric]

What does this PR do

Previous implementations of {get,set}regset are incorrect when register set value is anything other than NT_PRSTATUS. This PR adds the ability to return corresponding struct to the value using type parameters S: RegisterSet, so the behaviour is correct.

Added NT_PRSTATUS as well as NT_PRFPREG types so we can also probe floating-point registers. Other register sets may be inspected as well, such as arch-specific vector registers, but I don't have a use case right now, and it is easy to implement afterwards with the framework in this PR.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

This is an improvement over #2044, so changelog remains the same.

[TheJonny]

It could be more direct, if the RegisterSet would be implemented on the actual register set types. but the current suggestion has the advantage of having names one could recognize from the man page.

[hack3ric]

Sorry for the late response. Reviews have been addressed.

It could be more direct, if the RegisterSet would be implemented on the actual register set types. but the current suggestion has the advantage of having names one could recognize from the man page.

I'd add that it could be confusing to implement RegisterSet on actual register set types. user_regs_struct may be universal, but every platform has its own floating-pointer register space types (see impl RegisterSet for NT_PRFPREG).

[SteveLauC]

Thanks!