HvsockConn shutdown, and .IsClosed() function

Corrected `HvsockConn` `CloseRead`/`Write` to check if the socket
has been closed before attempting to shutdown the socket for
reading or writing.
Additionally, `shutdown` was not respecting whether to shutdown reading
or writing, and only shutting down the socket for reading.

Added `IsClosed` function to `win32File` (and therefore `win32Pipe` and
`win32MessageBytePipe`) and `HvsockConn` to check if the socket/pipe
has been closed already.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

file.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package winio
@@ -143,6 +144,11 @@ func (f *win32File) Close() error {
 	return nil
 }
 
+// IsClosed checks if the file has been closed
+func (f *win32File) IsClosed() bool {
+	return f.closing.isSet()
+}
+
 // prepareIo prepares for a new IO operation.
 // The caller must call f.wg.Done() when the IO is finished, prior to Close() returning.
 func (f *win32File) prepareIo() (*ioOperation, error) {