fix(dialer): correct inverted setMark on FreeBSD/OpenBSD

Fix setMark early-return conditions that were inverted relative to
Linux: on FreeBSD (SO_USER_COOKIE) and OpenBSD (SO_RTABLE), a
non-zero mark was short-circuited instead of applied, disabling
socket marking entirely.

Also fix GetClientIP to trim leading whitespace from X-Forwarded-For
entries (per RFC 7239), and fix Body.Read to subtract len(b) instead
of n from recordSize when a single read exceeds the remaining quota.

Add unit tests for internal/net/ (addr, dialer, http, ip, net, pipe,
transport, resolve, proxyproto, udp).

internal/net/dialer/dialer_linux_test.go

@@ -0,0 +1,40 @@
+package dialer
+
+import (
+	"testing"
+)
+
+func Test_bindDevice(t *testing.T) {
+	// Empty interface name - no-op
+	err := bindDevice("tcp", "127.0.0.1:80", 0, "")
+	if err != nil {
+		t.Errorf("expected nil for empty ifce, got %v", err)
+	}
+
+	// Non-global unicast IP (loopback) with port - skips binding
+	err = bindDevice("tcp", "127.0.0.1:80", 0, "lo")
+	if err != nil {
+		t.Errorf("expected nil for non-global unicast, got %v", err)
+	}
+
+	// ::1 (IPv6 loopback) is also non-global unicast
+	err = bindDevice("tcp", "[::1]:80", 0, "lo")
+	if err != nil {
+		t.Errorf("expected nil for IPv6 loopback, got %v", err)
+	}
+}
+
+func Test_setMark(t *testing.T) {
+	// mark=0 is no-op
+	err := setMark(0, 0)
+	if err != nil {
+		t.Errorf("expected nil for mark=0, got %v", err)
+	}
+
+	// Non-zero mark should succeed or fail based on fd validity
+	// We test with fd=0 which is stdin (should succeed with SO_MARK)
+	err = setMark(0, 1)
+	if err != nil {
+		t.Logf("setMark with fd=0: %v (expected on some systems)", err)
+	}
+}