Add support for Packet MMAP

[DBLouis]

This is WIP

A bit of guidance would be greatly appreciated :)

sunfishcode/linux-raw-sys#103

[DBLouis]

What is the difference between #[cfg(linux_kernel)] and #[cfg(target_os = "linux"]? When should I use which?

[sunfishcode]

#[cfg(linux_kernel)] includes Android, while #[cfg(target_os = "linux")] does not. linux_kernel etc. are defined in the build.rs file.

[DBLouis]

I wonder if it is worth it to backport my changes to linux-raw-sys to an earlier new release like 0.4.13 instead of 0.6.4

[sunfishcode]

You can submit a PR for 0.4.x if you like, but if you just care about docs.rs, 0.6.x is sufficient.

[DBLouis]

You can submit a PR for 0.4.x if you like, but if you just care about docs.rs, 0.6.x is sufficient.

Sorry I meant the changes in this sunfishcode/linux-raw-sys@783bd07.
I need those here obviously, but maybe there is no need to require updating the dependency on linux-raw-sys all the way to 0.6.x.

[DBLouis]

I have added an example high-level safe "wrapper" in examples/packet/inner.rs. I'm thinking that is could be moved to it's own crate (e.g. rustix-packet-mmap)