Sockets returned by wxSocket::Accept() are non-blocking by default and
the only way to use them safely in worker threads is by switching them
to the blocking mode by calling SetFlags(wxSOCKET_BLOCK).
However this didn't work correctly since at least 2.8 days, as turning
wxSOCKET_BLOCK on didn't unregister the socket from the event loop, with
which it had been registered on creation. Fix this by doing this now,
which ensures that the main thread doesn't get any notifications about
the socket if it's used, in a blocking way, in a worker thread.
Note that making the new socket blocking after accpeting is still pretty
inefficient and pre-creating the socket as blocking and using
AcceptWith() is still preferable, but at least it does work now.
Closes#12886.
73f0c9dff8