do not ignore write error for any output size
The previous change was incomplete.
While it makes emacs --batch detect the vast majority of stdout
write failures, errors were still ignored whenever the output size is
k * (BUFSIZ+1) - 4. E.g., on a system with BUFSIZ of 4096,
$ emacs --batch --eval '(print (format "%4093d" 0))' > /dev/full \
&& echo FAIL: ignored write error
FAIL: ignored write error
$ emacs --batch --eval '(print (format "%20481d" 0))' > /dev/full \
&& echo FAIL: ignored write error
FAIL: ignored write error
* emacs.c (Fkill_emacs): Also test ferror. (Bug#9574)