bug#1082: 23.0.60; read-char unexpectedly halts execution of script

[Chong Yidong <cyd@stupidchicken.com>]

> Let filter.el consist of the forms:
>
>    (defun bc-filter (proc string)
>      (message "%s" string))
>
>    (message "starting")
>    (setq bc (start-process "bc" nil "/usr/bin/bc"))
>    (set-process-filter bc 'bc-filter)
>
>    (while t
>      (let ((char (read-char nil nil 0.1)))
>        (message "char: %s" char)))
>
> Where "/usr/bin/bc" is the GNU arbitrary precision calculator. Now:
>
>    mt-computer:~ mt$ emacs --script filter.el
>    starting
>    mt-computer:~ mt$
>
> I expect non-termination in this case, as in eval-buffer on filter.el.

This broke when SYNC_INPUT became the default; it works when Emacs is
recompiled without SYNC_INPUT, like Emacs 22.

I don't know what the specific cause of failure is, however.  Maybe the
SYNC_INPUT code should only be active when Emacs is run interactively.

Thoughts?