fix(s2n-quic-xdp): use unix-specific traits for better compat

[camshaft]

Description of changes:

I ran into a couple of incompatibilities with our current MSRV (1.63) and the XDP code. Namely:

• the AsRawFd trait was exposed in a generic location (std::os) in a later version. However, the same trait is still available in std::os::unix so we can just use that since XDP is always going to be Linux-specific.
• The 1.63 rustc complains about nested unsafe blocks in the libc macro so I just opted to make the caller responsible for the unsafe block.
• The structs with raw pointers needed Sync and Send impls, which are safe according to the inline comments.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.