Diagnose and guard against write errors dealing with config.status.

Diagnose and guard against write errors dealing with config.status.

The general idea is this: all write failures from `configure'
writing `config.status' are indicated by $ac_write_error, which
is only checked at the end.  This is safe because config.status
code is not executed before the file is complete.  Other write
failures, be they inside config.status, or in sub shell/awk
scripts spawned from configure or config.status, typically need
earlier checking, as their results are used right afterwards.
* lib/autoconf/status.m4 (AC_OUTPUT): Initialize `ac_write_fail'
before writing config.status, check afterwards.
(_AC_OUTPUT_FILES_PREPARE, _AC_OUTPUT_FILE)
(_AC_OUTPUT_HEADERS_PREPARE,_AC_OUTPUT_CONFIG_STATUS):
Set `ac_write_error' for write failures to config.status.  Barf
upon write failures to temporary files.
Adjust note about closing and reopening the here-document.
(_AC_OUTPUT_HEADER, _AC_OUTPUT_LINK, _AC_OUTPUT_COMMAND)
(_AC_OUTPUT_MAIN_LOOP): Likewise, adjust note about closing and
reopening the here-document.
* tests/torture.at (AC_CONFIG_FILES, HEADERS, LINKS and COMMANDS):
Ensure `ac_write_error' does not escape into config.status.
Also, add a couple of code paths not yet exercised in the test
suite: a config file with input from stdin, and a config header
output to stdout.
Suggestion for catching write errors by Bruno Haible.