Improve ergonomics between SockaddrIn and underlying libc type

[pronebird]

What does this PR do

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

• Add Into<libc::sockaddr_in6> for SockaddrIn6

• Add Into<libc::sockaddr_in> for SockaddrIn

My typical use case for these Into traits is when I am bridging from safe world into the world of pain (C). For example, presently I have to go over as_ref() to access the underlying sockaddr_inX . This is a bit verbose for my liking, so I figured it would be great to have into() to do the job for me. Usually From is preferred but I am not certain if we want that, hence I have implemented Into but can easily flip to From if that makes sense.

let addr_sa = SockaddrIn6::from(SocketAddrV6::new(addr, 0, 0, 0));
let mut req: in6_aliasreq = unsafe { mem::zeroed() };
-req.addr = *addr_sa.as_ref();
+req.addr = addr_sa.into();

I also noticed that repr(transparent) is set on these structs and naturally I thought I could use addr_sa as sockaddr_in6 but that doesn't seem to compile.

[SteveLauC]

Would you like to do:

impl From<libc::sockaddr_in> for SockaddrIn
impl From<SockaddrIn> for libc::sockaddrin

impl From<libc::sockaddr_in6> for SockaddrIn6
impl From<SockaddrIn6> for libc::sockaddrin6

So that we can do roundtrips and also make clippy happy:)

[pronebird]

Done.

[SteveLauC]

This is indeed something we should do to allow Nix to be used in such scenarios, see #2327, and thanks for your interest in contributing to Nix!

[pronebird]

CI failure in test_sigsuspend is probably unrelated.

[SteveLauC]

Thanks! That CI failure is not related:)