In addition to unblocking and registering the socket, also support using
this function to make the socket blocking and unregistering it from the
event loop, if its flags include wxSOCKET_BLOCK.
This was already half-done by wxMSW, which took wxSOCKET_BLOCK presence
into account in its implementation, but not by the Unix implementation.
Now do it under all platforms, as this will be useful for switching a
previously non-blocking socket to blocking mode.
Finally, rename the function to better reflect what it really does.
See #12886.
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