chore(client): refine unsafe code restrictions for Linux ancillary features

Transition from a global forbid to a scoped deny for unsafe code to accommodate necessary low-level operations in the Linux receive implementation.

crates/irtt-client/src/lib.rs

@@ -2,6 +2,7 @@
     not(all(target_os = "linux", feature = "ancillary")),
     forbid(unsafe_code)
 )]
+#![cfg_attr(all(target_os = "linux", feature = "ancillary"), deny(unsafe_code))]
 
 mod client;
 mod config;

crates/irtt-client/src/receive/linux.rs

@@ -1,3 +1,4 @@
+#![allow(unsafe_code)]
 use std::{
     io, mem,
     net::{SocketAddr, UdpSocket},