Extend and rename wxSocketImpl::UnblockAndRegisterWithEventLoop()

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.

include/wx/msw/private/sockmsw.h

@@ -60,7 +60,7 @@ public:
 private:
     virtual void DoClose() wxOVERRIDE;
 
-    virtual void UnblockAndRegisterWithEventLoop() wxOVERRIDE
+    virtual void UpdateBlockingState() wxOVERRIDE
     {
         if ( GetSocketFlags() & wxSOCKET_BLOCK )
         {
@@ -74,6 +74,9 @@ private:
             // would result in data races and other unpleasantness.
             wxIoctlSocketArg_t trueArg = 1;
             ioctlsocket(m_fd, FIONBIO, &trueArg);
+
+            // Uninstall it in case it was installed before.
+            wxSocketManager::Get()->Uninstall_Callback(this);
         }
         else
         {