For unknown reasons, the pty will occasionally quit running. In these
cases, we still want to remove it so that a fresh one can be created. We
don't actually need this check because error messages from `zle` and
`zpty` are redirected to /dev/null.
One sure way to kill all currently running pty's is to run `exit` in a
subshell. Even without zsh-autosuggestions loaded, the following works:
% zmodload zsh/zpty
% zpty -b foo cat
% zpty -b bar cat
% zpty
(31689) bar: cat
(31666) foo: cat
% $(exit)
% zpty
(finished) bar: cat
(finished) foo: cat
