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>

hvsock.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package winio
@@ -252,15 +253,23 @@ func (conn *HvsockConn) Close() error {
 	return conn.sock.Close()
 }
 
+func (conn *HvsockConn) IsClosed() bool {
+	return conn.sock.IsClosed()
+}
+
 func (conn *HvsockConn) shutdown(how int) error {
-	err := syscall.Shutdown(conn.sock.handle, syscall.SHUT_RD)
+	if conn.IsClosed() {
+		return ErrFileClosed
+	}
+
+	err := syscall.Shutdown(conn.sock.handle, how)
 	if err != nil {
 		return os.NewSyscallError("shutdown", err)
 	}
 	return nil
 }
 
-// CloseRead shuts down the read end of the socket.
+// CloseRead shuts down the read end of the socket, preventing future read operations.
 func (conn *HvsockConn) CloseRead() error {
 	err := conn.shutdown(syscall.SHUT_RD)
 	if err != nil {
@@ -269,8 +278,8 @@ func (conn *HvsockConn) CloseRead() error {
 	return nil
 }
 
-// CloseWrite shuts down the write end of the socket, notifying the other endpoint that
-// no more data will be written.
+// CloseWrite shuts down the write end of the socket, preventing future write operations and
+// notifying the other endpoint that no more data will be written.
 func (conn *HvsockConn) CloseWrite() error {
 	err := conn.shutdown(syscall.SHUT_WR)
 	if err != nil {