Blocking issues on master

[Thomasdezeeuw]

This is mostly a note to myself.

These are blocking issue for a new release, either v0.4.x or v0.5, that are currently on master.

• After pr Allow for niche optimization on Unix platforms #222 was merged issue SockRef::from, Socket::sendfile and other functions that operate on arbitrary file descriptors or SOCKETs potentially should be unsafe #218 was made even worse as issue Unsoundness in <SockRef as From<&AsRawFd>>::from #229 shows. Either Allow for niche optimization on Unix platforms #222 must be reverted or SockRef::from, Socket::sendfile and other functions that operate on arbitrary file descriptors or SOCKETs potentially should be unsafe #218 solved before another release.