refactor(client): replace unsafe libc calls with nix for Linux ancillary data

Eliminate unsafe code blocks and manual pointer arithmetic in Linux-specific receive paths by utilizing nix for socket options and control message parsing. This improves safety and maintainability while preserving the existing kernel RX timestamp and traffic class functionality.

crates/irtt-client/Cargo.toml

@@ -13,14 +13,16 @@ categories = ["network-programming"]
 
 [dependencies]
 irtt-proto.workspace = true
-libc = { version = "0.2.189", optional = true }
 socket2 = { version = "0.6.5", features = ["all"] }
 thiserror = "2.0.20"
 tokio = { version = "1", default-features = false, features = ["rt", "net", "time", "sync"], optional = true}
 
+[target.'cfg(target_os = "linux")'.dependencies]
+nix = { version = "0.31", default-features = false, features = ["net", "uio"], optional = true }
+
 [features]
 default = []
-ancillary = ["dep:libc"]
+ancillary = ["dep:nix"]
 tokio = ["dep:tokio"]
 
 [package.metadata.docs.rs]