No documented way to detect terminal disconnection on getch

[Klaas van Aarsen]

We have a bug in the Angband code related to getch of ncurses (
https://github.com/angband/angband/pull/6559).
It turns out that getch returns ERR when the terminal connection is broken
(SIGHUP). The code currently interprets it as a timeout (set by halfdelay)
and spins on input, leaving a zombie process with 100% CPU.
The man page on getch only mentions that errno might be EINTR if the getch
call is interrupted, but nothing on a broken terminal connection.
How might we detect that the terminal connection is broken?