Cygwin: pty: Fix reading CONIN$ when stdin is not a pty.

Cygwin: pty: Fix reading CONIN$ when stdin is not a pty.

Previously, the pty master sends inputs to the pipe for cygwin app
even when pseudo console is activated if stdin is not the pty.
This causes the problem that key input is not sent to non cygwin
app even if the app opens CONIN$. This patch sets switch_to_nat_pipe
to true regardless whether stdin is the pty or not to allow that case.

https://cygwin.com/pipermail/cygwin/2023-April/253424.html

Reported-by: Wladislav Artsimovich <cygwin@frost.kiwi>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>