Kernel: Properly support the SO_BROADCAST socket option

POSIX requires that broadcast sends will only be allowed if the
SO_BROADCAST socket option was set on the socket.
Also, broadcast sends to protocols that do not support broadcast (like
TCP), should always fail.

Kernel/Net/Socket.h

@@ -156,6 +156,7 @@ protected:
     ucred m_origin { 0, 0, 0 };
     ucred m_acceptor { 0, 0, 0 };
     bool m_routing_disabled { false };
+    bool m_broadcast_allowed { false };
 
 private:
     virtual bool is_socket() const final { return true; }