LibIPC: Add Connection::is_open()

This will allow clients to see if the IPC socket is still open.

Userland/Libraries/LibIPC/Connection.h

@@ -132,6 +132,8 @@ public:
 
     virtual void die() { }
 
+    bool is_open() const { return m_socket->is_open(); }
+
 protected:
     Core::LocalSocket& socket() { return *m_socket; }