IPv4: Basic implementation of TCP socket shutdown

We can now participate in the TCP connection closing handshake. :^)
This implementation is definitely not complete and needs to handle a
bunch of other cases. But it's a huge improvement over not being able
to close connections at all.

Note that we hold on to pending-close sockets indefinitely, until they
are moved into the Closed state. This should also have a timeout but
that's still a FIXME. :^)

Fixes #428.

Kernel/Net/Socket.h

@@ -141,6 +141,9 @@ protected:
 
     virtual const char* class_name() const override { return "Socket"; }
 
+    virtual void shut_down_for_reading() {}
+    virtual void shut_down_for_writing() {}
+
     Role m_role { Role::None };
 
 protected: