net/SocketAddress: add IsV4Mapped()

src/net/SocketAddress.hxx

@@ -114,6 +114,12 @@ public:
 	gcc_pure
 	bool IsV6Any() const noexcept;
 
+	/**
+	 * Is this an IPv4 address mapped inside struct sockaddr_in6?
+	 */
+	gcc_pure
+	bool IsV4Mapped() const noexcept;
+
 	/**
 	 * Extract the port number.  Returns 0 if not applicable.
 	 */