Fix Kakoune client busy looping on SIGHUP

Pure clients never quitted when they got SIGHUP after recent changes
to add is_ok to UserInterface. run_client now tracks the UI state as
well and quits if the UI gets killed.

src/main.cc

@@ -526,7 +526,7 @@ int run_client(StringView session, StringView name, StringView client_init,
                             client_init, std::move(init_coord)};
         if (suspend)
             raise(SIGTSTP);
-        while (not client.exit_status())
+        while (not client.exit_status() and client.is_ui_ok())
             event_manager.handle_next_events(EventMode::Normal);
         return *client.exit_status();
     }

src/remote.cc

@@ -668,6 +668,11 @@ RemoteClient::RemoteClient(StringView session, StringView name, std::unique_ptr<
     }});
 }
 
+bool RemoteClient::is_ui_ok() const
+{
+    return m_ui->is_ok();
+}
+
 void send_command(StringView session, StringView command)
 {
     int sock = connect_to(session);

src/remote.hh

@@ -34,6 +34,7 @@ public:
                  int pid, const EnvVarMap& env_vars, StringView init_command,
                  Optional<BufferCoord> init_coord);
 
+    bool is_ui_ok() const;
     const Optional<int>& exit_status() const { return m_exit_status; }
 private:
     std::unique_ptr<UserInterface> m_ui;