Fix file descriptor being closed twice

This was leading to us closing fd that was in use by other routine and
thus causing problems in singlepoll code that should never have same fd
parked twice.

The cause was following:
If FileConn failed due to lack of free fd's for Dup operation we were
calling Close syscall to close the fd. This fd was previously passed to
os.NewFile which causes Golang to add finalizer for closing that fd.

If memory, file descriptor and connection pressure were right, Golang was
closing fd that was being parked on, which in turns meant that we could
get the same fd in the dial code, which would be parked again causing
the singlepoller to enter undefined state.

impl_unix.go

@@ -163,14 +163,15 @@ func dial(ctx context.Context, dialer net.Dialer, netw, addr string) (c net.Conn
 		}
 	}
 
+	// NOTE:XXX: never call syscall.Close on fd after os.NewFile
 	file = os.NewFile(uintptr(fd), filePrefix+strconv.Itoa(os.Getpid()))
+	fd = -1 // so we don't touch it, we handled the control to Golang with NewFile
 	if c, err = net.FileConn(file); err != nil {
-		syscall.Close(fd)
+		_ = file.Close() // shouldn't error either way
 		return nil, err
 	}
 
 	if err = file.Close(); err != nil {
-		syscall.Close(fd)
 		c.Close()
 		return nil, err
 	}