Fixes assertion failure (regression).

Allow creating a closed SocketMonitor instance.

Use WinSock errors on Windows.

Make a class that contains the formatting code and the buffer.

Using initializer list.

Not necessary, GLib will pick up the updated GPollFD automatically.

.. instead of calling g_main_loop_quit(), which may dead-lock.

This reverts commit f53dadcc, to
prepare for a better solution.

It seems that it is not safe to remove the item from std::list
while it's being iterated somewhere else.

This is a very simple quick fix to make things work
until some better solution is implemented.