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/IPv4Socket.h

@@ -46,6 +46,7 @@ public:
 
     static Lockable<HashTable<IPv4Socket*>>& all_sockets();
 
+    virtual void close() override;
     virtual KResult bind(const sockaddr*, socklen_t) override;
     virtual KResult connect(FileDescription&, const sockaddr*, socklen_t, ShouldBlock = ShouldBlock::Yes) override;
     virtual KResult listen(int) override;