Add support for FreeBSD

[bigsaltyfishes]

Constant and structure extract from <sys/socket.h>, <netlink/netlink.h> and <netlink/netlink_route.h>, some unsupported features are disabled for FreeBSD.

• set_no_enobufs and set_broadcast_error: System report protocol unsupported, disabled.
• connect: System report Os { code: 56, kind: Uncategorized, message: "Socket is already connected" , disabled.

[gemini-code-assist]

Summary of Changes

Hello @bigsaltyfishes, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request extends the library's compatibility to include the FreeBSD operating system. The changes involve carefully isolating platform-specific system calls and data structures, and conditionally disabling features that are not available or behave differently on FreeBSD, ensuring a robust and portable netlink implementation.

Highlights

• FreeBSD Support: Added initial support for the FreeBSD operating system, expanding the library's compatibility.
• Platform-Specific Definitions: Introduced a new sys module and ffi/freebsd.rs to define FreeBSD-specific constants and the sockaddr_nl structure, ensuring correct system call interfaces.
• Conditional Compilation: Implemented conditional compilation (#[cfg(target_os = "freebsd")]) to adapt netlink constants and disable features like connect, set_no_enobufs, and set_broadcast_error on FreeBSD, as they are reported as unsupported.
• Code Refactoring: Updated SocketAddr and Socket implementations to utilize the new platform-agnostic sockaddr_nl type and constants, improving code maintainability and portability.

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[gemini-code-assist]

Code Review

This pull request adds support for FreeBSD by introducing OS-specific constants and types, and by disabling unsupported features on FreeBSD using conditional compilation. The changes are well-structured for cross-platform compatibility. However, I've identified a critical issue in the new sys.rs module that would break compilation on non-FreeBSD platforms. Additionally, there are a couple of high-severity issues in socket.rs where getter functions for unsupported socket options were not disabled for FreeBSD, which would lead to runtime errors. I have provided suggestions to fix these issues.