multi: fix slow write/upload performance on Windows

multi: fix slow write/upload performance on Windows

Reset FD_WRITE by sending zero bytes which is permissible
and will be treated by implementations as successful send.

Without this we won't be notified in case a socket is still
writable if we already received such a notification and did
not send any data afterwards on the socket. This would lead
to waiting forever on a writable socket being writable again.

Assisted-by: Tommy Odom
Reviewed-by: Jay Satiro
Reviewed-by: Marcel Raad
Tested-by: tmkk on github
Bug: #6146
Closes #6245