Regression with stdin handling in non-interactive mode between 5.8 and 5.8.1

[Lyude Paul <lyude@redhat.com>]

Hi! I'm reporting this here because after some discussion in #zsh, it was
determined this is likely both a regression, and also isn't POSIX compliant.
Keep in mind I don't have as clear of an understanding of what's happening
below the hood here as I'd like, so I'm not 100% the subject line here is
correct. I've got plenty of examples to clarify though :)

Basically, what seems to be happening is that since 5.8.1 zsh no longer seems
to correctly handle input from stdin unless the terminal is in interactive
mode.

Here's a simple example script that demonstrates what I mean:

   printf '%s\n' 'echo Shell is $$' sh 'echo Shell is $$' | zsh

Running on zsh 5.8 returns:

   Shell is 70
   Shell is 71

Running on zsh 5.8.1 however, returns:

   Shell is 86396
   Shell is 86396

This can end up being an issue when trying to do things like (assuming sudo is
configured on this system to not request a password, and zsh is the default
shell):

   ssh foo <<- _EOF_
   whoami
   sudo -s
   whoami
   _EOF_

While that's maybe not the best way of doing such things in shell, I have
quite a number of scripts that rely on this working and have for quite some
time. According to llua from #zsh as well, this is also likely not POSIX
compliant according to:

   https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html (see the
   section labeled "INPUT FILES")

llua suggested that this breakage may have come from
e5cd2dd980302f328d232d933f646c3dc02828bf ("49290: Replace stdio for buffered
shell input."), which I've confirmed to be true by bisecting this locally. 

For reference: I originally reproduced this on Fedora 35, although I have a
feeling that probably doesn't matter too much here. If there's any other
information I can provide that would help with getting this fixed, don't
hesistate to ask. And thank you ahead of time!
-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat