Never use events for blocking sockets under Unix.

Events are not needed for this kind of sockets, using wxSOCKET_BLOCK is
supposed to ensure that calling socket IO operations blocks until the bytes
are read/written without dispatching any events.

See #17031.

src/unix/sockunix.cpp

@@ -90,6 +90,11 @@ wxSocketError wxSocketImplUnix::GetLastError() const
 
 void wxSocketImplUnix::DoEnableEvents(int flags, bool enable)
 {
+    // No events for blocking sockets, they should be usable from the other
+    // threads and the events only work for the sockets used by the main one.
+    if ( GetSocketFlags() & wxSOCKET_BLOCK )
+        return;
+
     wxSocketManager * const manager = wxSocketManager::Get();
     if (!manager)
         return;