Set UDP socket buffer sizes to prevent receive overflow

Under high-throughput forwarding, the kernel default 212KB receive
buffer overflows, silently dropping ~11% of UDP datagrams at transit
nodes. Add configurable recv_buf_size and send_buf_size to UdpConfig
(default 2MB each) using socket2 for pre-bind buffer configuration.
Startup log now reports actual buffer sizes granted by the kernel.

Requires net.core.rmem_max >= 2097152 on the host for the full 2MB
to take effect; otherwise the kernel silently clamps.

Cargo.toml

@@ -24,6 +24,7 @@ rtnetlink = "0.14"
 tokio = { version = "1", features = ["rt", "macros", "signal", "sync", "net", "time"] }
 futures = "0.3"
 simple-dns = "0.9"
+socket2 = { version = "0.5", features = ["all"] }
 
 [dev-dependencies]
 tempfile = "3.15"