Win32: detection of connect() errors in select(). On Windows, connect() errors are only reported via exceptfds descriptor set from select(). Previously exceptfds was set to NULL, and connect() errors were not detected at all, so connects to closed ports were waiting till a timeout occurred. Since ongoing connect() means that there will be a write event active, except descriptor set is copied from the write one. While it is possible to construct except descriptor set as a concatenation of both read and write descriptor sets, this looks unneeded. With this change, connect() errors are properly detected now when using select(). Note well that it is not possible to detect connect() errors with WSAPoll() (see https://daniel.haxx.se/blog/2012/10/10/wsapoll-is-broken/).