os, internal/poll: don't use splice with tty

os, internal/poll: don't use splice with tty

Also don't try to wait for a non-pollable FD.

Fixes #59041

Change-Id: Ife469d8738f2cc27c0beba223bdc8f8bc757b2a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/476335
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>