Change the semantics of AS_EXIT without argument.

Change the semantics of AS_EXIT without argument.

* lib/m4sugar/m4sh.m4 (_AS_EXIT_PREPARE): When defaulting, use $?
even if it is 0.
(AS_ERROR): Guarantee non-zero status.
* bin/autoconf.as (exit_missing_arg, getopt): Revert prior change;
we want non-zero status.
* tests/m4sh.at (AS@&t@_EXIT): Update test accordingly.
* doc/autoconf.texi (Common Shell Constructs) <AS_EXIT>: Update
the documentation.
<AS_SET_STATUS>: Don't overly restrict implementation.
(Printing Messages) <AC_MSG_ERROR>: Describe better default.
Suggestions by Paolo Bonzini and Ralf Wildenhues.

Signed-off-by: Eric Blake <ebb9@byu.net>