Add macos support

[ovo-Tim]

#95
Adds macOS host support by gating Linux-only features and dependencies behind cfg(target_os = "linux").

Changes

• Make Landlock optional on non-Linux (no-op on macOS) and move landlock dependency to Linux-only target deps.
• Gate Linux-only nix usage (prctl, inotify, major/minor) and add non-Linux-safe behavior where needed.
• Refactor devices.rs into clean platform-specific implementations; non-Linux now returns a clear unsupported error for device attachment.
• Update README to document macOS support and clarify Linux-only features (Landlock, litterbox device).

Do not merge for now. The test is still going.

[ovo-Tim]

It's good to go now

[Gerharddc]

Thanks for the PR! As indicated in my comments, I would prefer if this change be much smaller and not have so much platform-conditional code. I believe that is possible by approaching things differently.

[Gerharddc]

Please rebase this on #101 and check how well it works on macOS. This change should make support much easier because now there is a dedicated "lbx-init" binary which goes inside the container and can easily be cross-compiled

[ovo-Tim]

Good to go again 🎉

[Gerharddc]

Sorry for the delay, but great to see the Linux lbx-init binary now works on MacOS! I've now merged my large PR after making a few more tweaks. Unfortunately it seems you will now have to rebase again after those changes, sorry about that