bug#30762: 27.0.50; Apparent regression in process.c caused by checking pty_flag before calling setsid

[Paul Eggert]

Eli Zaretskii wrote:
> This change is on the emacs-26 branch as well, so if we want to
> restore previous code, we should do it on emacs-26, not only on
> master.

I installed the fix into emacs-26 and merged it into master. Thanks, Thomas, for 
reporting it and for tracking down the fix.